// -*- mode:doc; -*-
  // vim: set syntax=asciidoc:
  
  === Infrastructure for packages with specific build systems
  
  By 'packages with specific build systems' we mean all the packages
  whose build system is not one of the standard ones, such as
  'autotools' or 'CMake'. This typically includes packages whose build
  system is based on hand-written Makefiles or shell scripts.
  
  [[generic-package-tutorial]]
  
  ==== +generic-package+ tutorial
  
  ------------------------------
  01: ################################################################################
  02: #
  03: # libfoo
  04: #
  05: ################################################################################
  06:
  07: LIBFOO_VERSION = 1.0
  08: LIBFOO_SOURCE = libfoo-$(LIBFOO_VERSION).tar.gz
  09: LIBFOO_SITE = http://www.foosoftware.org/download
  10: LIBFOO_LICENSE = GPLv3+
  11: LIBFOO_LICENSE_FILES = COPYING
  12: LIBFOO_INSTALL_STAGING = YES
  13: LIBFOO_CONFIG_SCRIPTS = libfoo-config
  14: LIBFOO_DEPENDENCIES = host-libaaa libbbb
  15:
  16: define LIBFOO_BUILD_CMDS
  17:	$(MAKE) $(TARGET_CONFIGURE_OPTS) -C $(@D) all
  18: endef
  19:
  20: define LIBFOO_INSTALL_STAGING_CMDS
  21:	$(INSTALL) -D -m 0755 $(@D)/libfoo.a $(STAGING_DIR)/usr/lib/libfoo.a
  22:	$(INSTALL) -D -m 0644 $(@D)/foo.h $(STAGING_DIR)/usr/include/foo.h
  23:	$(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(STAGING_DIR)/usr/lib
  24: endef
  25:
  26: define LIBFOO_INSTALL_TARGET_CMDS
  27:	$(INSTALL) -D -m 0755 $(@D)/libfoo.so* $(TARGET_DIR)/usr/lib
  28:	$(INSTALL) -d -m 0755 $(TARGET_DIR)/etc/foo.d
  29: endef
  30:
  31: define LIBFOO_USERS
  32:	foo -1 libfoo -1 * - - - LibFoo daemon
  33: endef
  34:
  35: define LIBFOO_DEVICES
  36:	/dev/foo  c  666  0  0	42  0  -  -  -
  37: endef
  38:
  39: define LIBFOO_PERMISSIONS
  40:	/bin/foo  f  4755  foo  libfoo	 -  -  -  -  -
  41: endef
  42:
  43: $(eval $(generic-package))
  --------------------------------
  
  The Makefile begins on line 7 to 11 with metadata information: the
  version of the package (+LIBFOO_VERSION+), the name of the
  tarball containing the package (+LIBFOO_SOURCE+) (xz-ed tarball recommended)
  the Internet location at which the tarball can be downloaded from
  (+LIBFOO_SITE+), the license (+LIBFOO_LICENSE+) and file with the
  license text (+LIBFOO_LICENSE_FILES+). All variables must start with
  the same prefix, +LIBFOO_+ in this case. This prefix is always the
  uppercased version of the package name (see below to understand where
  the package name is defined).
  
  On line 12, we specify that this package wants to install something to
  the staging space. This is often needed for libraries, since they must
  install header files and other development files in the staging space.
  This will ensure that the commands listed in the
  +LIBFOO_INSTALL_STAGING_CMDS+ variable will be executed.
  
  On line 13, we specify that there is some fixing to be done to some
  of the 'libfoo-config' files that were installed during
  +LIBFOO_INSTALL_STAGING_CMDS+ phase.
  These *-config files are executable shell script files that are
  located in '$(STAGING_DIR)/usr/bin' directory and are executed
  by other 3rd party packages to find out the location and the linking
  flags of this particular package.
  
  The problem is that all these *-config files by default give wrong,
  host system linking flags that are unsuitable for cross-compiling.
  
  For example:	'-I/usr/include' instead of '-I$(STAGING_DIR)/usr/include'
  or:		'-L/usr/lib' instead of '-L$(STAGING_DIR)/usr/lib'
  
  So some sed magic is done to these scripts to make them give correct
  flags.
  The argument to be given to +LIBFOO_CONFIG_SCRIPTS+ is the file name(s)
  of the shell script(s) needing fixing. All these names are relative to
  '$(STAGING_DIR)/usr/bin' and if needed multiple names can be given.
  
  In addition, the scripts listed in +LIBFOO_CONFIG_SCRIPTS+ are removed
  from +$(TARGET_DIR)/usr/bin+, since they are not needed on the target.
  
  .Config script: 'divine' package
  ================================
  Package divine installs shell script '$(STAGING_DIR)/usr/bin/divine-config'.
  
  So its fixup would be:
  
  --------------------------------
  DIVINE_CONFIG_SCRIPTS = divine-config
  --------------------------------
  ================================
  
  .Config script: 'imagemagick' package:
  ================================
  Package imagemagick installs the following scripts:
  '$(STAGING_DIR)/usr/bin/{Magick,Magick++,MagickCore,MagickWand,Wand}-config'
  
  So it's fixup would be:
  
  --------------------------------
  IMAGEMAGICK_CONFIG_SCRIPTS = \
     Magick-config Magick++-config \
     MagickCore-config MagickWand-config Wand-config
  --------------------------------
  ================================
  
  On line 14, we specify the list of dependencies this package relies
  on. These dependencies are listed in terms of lower-case package names,
  which can be packages for the target (without the +host-+
  prefix) or packages for the host (with the +host-+) prefix).
  Buildroot will ensure that all these packages are built and installed
  'before' the current package starts its configuration.
  
  The rest of the Makefile, lines 16..29, defines what should be done
  at the different steps of the package configuration, compilation and
  installation.
  +LIBFOO_BUILD_CMDS+ tells what steps should be performed to
  build the package. +LIBFOO_INSTALL_STAGING_CMDS+ tells what
  steps should be performed to install the package in the staging space.
  +LIBFOO_INSTALL_TARGET_CMDS+ tells what steps should be
  performed to install the package in the target space.
  
  All these steps rely on the +$(@D)+ variable, which
  contains the directory where the source code of the package has been
  extracted.
  
  On lines 31..43, we define a user that is used by this package (e.g.
  to run a daemon as non-root) (+LIBFOO_USERS+).
  
  On line 35..37, we define a device-node file used by this package
  (+LIBFOO_DEVICES+).
  
  On line 39..41, we define the permissions to set to specific files
  installed by this package (+LIBFOO_PERMISSIONS+).
  
  Finally, on line 43, we call the +generic-package+ function, which
  generates, according to the variables defined previously, all the
  Makefile code necessary to make your package working.
  
  [[generic-package-reference]]
  
  ==== +generic-package+ reference
  
  There are two variants of the generic target. The +generic-package+ macro is
  used for packages to be cross-compiled for the target. The
  +host-generic-package+ macro is used for host packages, natively compiled
  for the host. It is possible to call both of them in a single +.mk+
  file: once to create the rules to generate a target
  package and once to create the rules to generate a host package:
  
  ----------------------
  $(eval $(generic-package))
  $(eval $(host-generic-package))
  ----------------------
  
  This might be useful if the compilation of the target package requires
  some tools to be installed on the host. If the package name is
  +libfoo+, then the name of the package for the target is also
  +libfoo+, while the name of the package for the host is
  +host-libfoo+. These names should be used in the DEPENDENCIES
  variables of other packages, if they depend on +libfoo+ or
  +host-libfoo+.
  
  The call to the +generic-package+ and/or +host-generic-package+ macro *must* be
  at the end of the +.mk+ file, after all variable definitions.
  
  For the target package, the +generic-package+ uses the variables defined by
  the .mk file and prefixed by the uppercased package name:
  +LIBFOO_*+. +host-generic-package+ uses the +HOST_LIBFOO_*+ variables. For
  'some' variables, if the +HOST_LIBFOO_+ prefixed variable doesn't
  exist, the package infrastructure uses the corresponding variable
  prefixed by +LIBFOO_+. This is done for variables that are likely to
  have the same value for both the target and host packages. See below
  for details.
  
  The list of variables that can be set in a +.mk+ file to give metadata
  information is (assuming the package name is +libfoo+) :
  
  * +LIBFOO_VERSION+, mandatory, must contain the version of the
    package. Note that if +HOST_LIBFOO_VERSION+ doesn't exist, it is
    assumed to be the same as +LIBFOO_VERSION+. It can also be a
    revision number, branch or tag for packages that are fetched
    directly from their revision control system. +
    Examples: +
      +LIBFOO_VERSION = 0.1.2+ +
      +LIBFOO_VERSION = cb9d6aa9429e838f0e54faa3d455bcbab5eef057+ +
      +LIBFOO_VERSION = stable+
  
  * +LIBFOO_SOURCE+ may contain the name of the tarball of the package,
    which Buildroot will use to download the tarball from
    +LIBFOO_SITE+. If +HOST_LIBFOO_SOURCE+ is not specified, it defaults
    to +LIBFOO_SOURCE+. If none are specified, then the value is assumed
    to be +libfoo-$(LIBFOO_VERSION).tar.gz+. +
    Example: +LIBFOO_SOURCE = foobar-$(LIBFOO_VERSION).tar.bz2+
  
  * +LIBFOO_PATCH+ may contain a space-separated list of patch file
    names, that Buildroot will download and apply to the package source
    code. If an entry contains +://+, then Buildroot will assume it is a
    full URL and download the patch from this location. Otherwise,
    Buildroot will assume that the patch should be downloaded from
    +LIBFOO_SITE+. If +HOST_LIBFOO_PATCH+ is not specified, it defaults
    to +LIBFOO_PATCH+. Note that patches that are included in Buildroot
    itself use a different mechanism: all files of the form
    +*.patch+ present in the package directory inside
    Buildroot will be applied to the package after extraction (see
    xref:patch-policy[patching a package]). Finally, patches listed in
    the +LIBFOO_PATCH+ variable are applied _before_ the patches stored
    in the Buildroot package directory.
  
  * +LIBFOO_SITE+ provides the location of the package, which can be a
    URL or a local filesystem path. HTTP, FTP and SCP are supported URL
    types for retrieving package tarballs. In these cases don't include a
    trailing slash: it will be added by Buildroot between the directory
    and the filename as appropriate. Git, Subversion, Mercurial,
    and Bazaar are supported URL types for retrieving packages directly
    from source code management systems. There is a helper function to make
    it easier to download source tarballs from GitHub (refer to
    xref:github-download-url[] for details). A filesystem path may be used
    to specify either a tarball or a directory containing the package
    source code. See +LIBFOO_SITE_METHOD+ below for more details on how
    retrieval works. +
    Note that SCP URLs should be of the form
    +scp://[user@]host:filepath+, and that filepath is relative to the
    user's home directory, so you may want to prepend the path with a
    slash for absolute paths:
    +scp://[user@]host:/absolutepath+. +
    If +HOST_LIBFOO_SITE+ is not specified, it defaults to
    +LIBFOO_SITE+.
    Examples: +
      +LIBFOO_SITE=http://www.libfoosoftware.org/libfoo+ +
      +LIBFOO_SITE=http://svn.xiph.org/trunk/Tremor+ +
      +LIBFOO_SITE=/opt/software/libfoo.tar.gz+ +
      +LIBFOO_SITE=$(TOPDIR)/../src/libfoo+
  
  * +LIBFOO_EXTRA_DOWNLOADS+ is a space-separated list of additional
    files that Buildroot should download. If an entry contains +://+
    then Buildroot will assume it is a complete URL and will download
    the file using this URL. Otherwise, Buildroot will assume the file
    to be downloaded is located at +LIBFOO_SITE+. Buildroot will not do
    anything with those additional files, except download them: it will
    be up to the package recipe to use them from +$(BR2_DL_DIR)+.
  
  * +LIBFOO_SITE_METHOD+ determines the method used to fetch or copy the
    package source code. In many cases, Buildroot guesses the method
    from the contents of +LIBFOO_SITE+ and setting +LIBFOO_SITE_METHOD+
    is unnecessary. When +HOST_LIBFOO_SITE_METHOD+ is not specified, it
    defaults to the value of +LIBFOO_SITE_METHOD+. +
    The possible values of +LIBFOO_SITE_METHOD+ are:
    ** +wget+ for normal FTP/HTTP downloads of tarballs. Used by
       default when +LIBFOO_SITE+ begins with +http://+, +https://+ or
       +ftp://+.
    ** +scp+ for downloads of tarballs over SSH with scp. Used by
       default when +LIBFOO_SITE+ begins with +scp://+.
    ** +svn+ for retrieving source code from a Subversion repository.
       Used by default when +LIBFOO_SITE+ begins with +svn://+. When a
       +http://+ Subversion repository URL is specified in
       +LIBFOO_SITE+, one 'must' specify +LIBFOO_SITE_METHOD=svn+.
       Buildroot performs a checkout which is preserved as a tarball in
       the download cache; subsequent builds use the tarball instead of
       performing another checkout.
    ** +cvs+ for retrieving source code from a CVS repository.
       Used by default when +LIBFOO_SITE+ begins with +cvs://+.
       The downloaded source code is cached as with the +svn+ method.
       Anonymous pserver mode is assumed otherwise explicitly defined
       on +LIBFOO_SITE+. Both
       +LIBFOO_SITE=cvs://libfoo.net:/cvsroot/libfoo+ and
       +LIBFOO_SITE=cvs://:ext:libfoo.net:/cvsroot/libfoo+
       are accepted, on the former anonymous pserver access mode is
       assumed.
       +LIBFOO_SITE+ 'must' contain the source URL as well as the remote
       repository directory. The module is the package name.
       +LIBFOO_VERSION+ is 'mandatory' and 'must' be a tag, a branch, or
       a date (e.g. "2014-10-20", "2014-10-20 13:45", "2014-10-20
       13:45+01" see "man cvs" for further details).
    ** +git+ for retrieving source code from a Git repository. Used by
       default when +LIBFOO_SITE+ begins with +git://+. The downloaded
       source code is cached as with the +svn+
       method.
    ** +hg+ for retrieving source code from a Mercurial repository. One
       'must' specify +LIBFOO_SITE_METHOD=hg+ when +LIBFOO_SITE+
       contains a Mercurial repository URL. The downloaded source code
       is cached as with the +svn+ method.
    ** +bzr+ for retrieving source code from a Bazaar repository. Used
       by default when +LIBFOO_SITE+ begins with +bzr://+. The
       downloaded source code is cached as with the +svn+ method.
    ** +file+ for a local tarball. One should use this when
       +LIBFOO_SITE+ specifies a package tarball as a local filename.
       Useful for software that isn't available publicly or in version
       control.
    ** +local+ for a local source code directory. One should use this
       when +LIBFOO_SITE+ specifies a local directory path containing
       the package source code. Buildroot copies the contents of the
       source directory into the package's build directory.
  
  * +LIBFOO_GIT_SUBMODULES+, when +LIBFOO_SITE_METHOD=git+, will create
    an archive with the git submodules in the repository.
    Note that we try not to use such git submodules when they contain
    bundled libraries, in which case we prefer to use those libraries
    from their own package.
  
  * +LIBFOO_STRIP_COMPONENTS+ is the number of leading components
    (directories) that tar must strip from file names on extraction.
    The tarball for most packages has one leading component named
    "<pkg-name>-<pkg-version>", thus Buildroot passes
    --strip-components=1 to tar to remove it.
    For non-standard packages that don't have this component, or
    that have more than one leading component to strip, set this
    variable with the value to be passed to tar. Default: 1.
  
  * +LIBFOO_EXCLUDES+ is a space-separated list of patterns to exclude
    when extracting the archive. Each item from that list is passed as
    a tar's +--exclude+ option. By default, empty.
  
  * +LIBFOO_DEPENDENCIES+ lists the dependencies (in terms of package
    name) that are required for the current target package to
    compile. These dependencies are guaranteed to be compiled and
    installed before the configuration of the current package starts. In
    a similar way, +HOST_LIBFOO_DEPENDENCIES+ lists the dependencies for
    the current host package.
  
  * +LIBFOO_PATCH_DEPENDENCIES+ lists the dependencies (in terms of
    package name) that are required for the current package to be
    patched. These dependencies are guaranteed to be extracted and
    patched before the current package is patched. In a similar way,
    +HOST_LIBFOO_PATCH_DEPENDENCIES+ lists the dependencies for the
    current host package.
    This is seldom used; usually, +LIBFOO_DEPENDENCIES+ is what you
    really want to use.
  
  * +LIBFOO_PROVIDES+ lists all the virtual packages +libfoo+ is an
    implementation of. See xref:virtual-package-tutorial[].
  
  * +LIBFOO_INSTALL_STAGING+ can be set to +YES+ or +NO+ (default). If
    set to +YES+, then the commands in the +LIBFOO_INSTALL_STAGING_CMDS+
    variables are executed to install the package into the staging
    directory.
  
  * +LIBFOO_INSTALL_TARGET+ can be set to +YES+ (default) or +NO+. If
    set to +YES+, then the commands in the +LIBFOO_INSTALL_TARGET_CMDS+
    variables are executed to install the package into the target
    directory.
  
  * +LIBFOO_INSTALL_IMAGES+ can be set to +YES+ or +NO+ (default). If
    set to +YES+, then the commands in the +LIBFOO_INSTALL_IMAGES_CMDS+
    variable are executed to install the package into the images
    directory.
  
  * +LIBFOO_CONFIG_SCRIPTS+ lists the names of the files in
    '$(STAGING_DIR)/usr/bin' that need some special fixing to make them
    cross-compiling friendly. Multiple file names separated by space can
    be given and all are relative to '$(STAGING_DIR)/usr/bin'. The files
    listed in +LIBFOO_CONFIG_SCRIPTS+ are also removed from
    +$(TARGET_DIR)/usr/bin+ since they are not needed on the target.
  
  * +LIBFOO_DEVICES+ lists the device files to be created by Buildroot
    when using the static device table. The syntax to use is the
    makedevs one. You can find some documentation for this syntax in the
    xref:makedev-syntax[]. This variable is optional.
  
  * +LIBFOO_PERMISSIONS+ lists the changes of permissions to be done at
    the end of the build process. The syntax is once again the makedevs one.
    You can find some documentation for this syntax in the xref:makedev-syntax[].
    This variable is optional.
  
  * +LIBFOO_USERS+ lists the users to create for this package, if it installs
    a program you want to run as a specific user (e.g. as a daemon, or as a
    cron-job). The syntax is similar in spirit to the makedevs one, and is
    described in the xref:makeuser-syntax[]. This variable is optional.
  
  * +LIBFOO_LICENSE+ defines the license (or licenses) under which the package
    is released.
    This name will appear in the manifest file produced by +make legal-info+.
    If the license appears in xref:legal-info-list-licenses[the following list],
    use the same string to make the manifest file uniform.
    Otherwise, describe the license in a precise and concise way, avoiding
    ambiguous names such as +BSD+ which actually name a family of licenses.
    This variable is optional. If it is not defined, +unknown+ will appear in
    the +license+ field of the manifest file for this package. +
    The expected format for this variable must comply with the following rules:
    ** If different parts of the package are released under different
    licenses, then +comma+ separate licenses (e.g. +`LIBFOO_LICENSE =
    GPLv2+, LGPLv2.1+`+). If there is clear distinction between which
    component is licensed under what license, then annotate the license
    with that component, between parenthesis (e.g. +`LIBFOO_LICENSE =
    GPLv2+ (programs), LGPLv2.1+ (libraries)`+).
    ** If the package is dual licensed, then separate licenses with the
    +or+ keyword (e.g. +`LIBFOO_LICENSE = AFLv2.1 or GPLv2+`+).
  
  * +LIBFOO_LICENSE_FILES+ is a space-separated list of files in the package
    tarball that contain the license(s) under which the package is released.
    +make legal-info+ copies all of these files in the +legal-info+ directory.
    See xref:legal-info[] for more information.
    This variable is optional. If it is not defined, a warning will be produced
    to let you know, and +not saved+ will appear in the +license files+ field
    of the manifest file for this package.
  
  * +LIBFOO_ACTUAL_SOURCE_TARBALL+ only applies to packages whose
    +LIBFOO_SITE+ / +LIBTOO_SOURCE+ pair points to an archive that does
    not actually contain source code, but binary code. This a very
    uncommon case, only known to apply to external toolchains which come
    already compiled, although theoretically it might apply to other
    packages. In such cases a separate tarball is usually available with
    the actual source code. Set +LIBFOO_ACTUAL_SOURCE_TARBALL+ to the
    name of the actual source code archive and Buildroot will download
    it and use it when you run +make legal-info+ to collect
    legally-relevant material.  Note this file will not be downloaded
    during regular builds nor by +make source+.
  
  * +LIBFOO_ACTUAL_SOURCE_SITE+ provides the location of the actual
    source tarball. The default value is +LIBFOO_SITE+, so you don't
    need to set this variable if the binary and source archives are
    hosted on the same directory.  If +LIBFOO_ACTUAL_SOURCE_TARBALL+ is
    not set, it doesn't make sense to define
    +LIBFOO_ACTUAL_SOURCE_SITE+.
  
  * +LIBFOO_REDISTRIBUTE+ can be set to +YES+ (default) or +NO+ to indicate if
    the package source code is allowed to be redistributed. Set it to +NO+ for
    non-opensource packages: Buildroot will not save the source code for this
    package when collecting the +legal-info+.
  
  * +LIBFOO_FLAT_STACKSIZE+ defines the stack size of an application built into
    the FLAT binary format. The application stack size on the NOMMU architecture
    processors can't be enlarged at run time. The default stack size for the
    FLAT binary format is only 4k bytes. If the application consumes more stack,
    append the required number here.
  
  The recommended way to define these variables is to use the following
  syntax:
  
  ----------------------
  LIBFOO_VERSION = 2.32
  ----------------------
  
  Now, the variables that define what should be performed at the
  different steps of the build process.
  
  * +LIBFOO_EXTRACT_CMDS+ lists the actions to be performed to extract
    the package. This is generally not needed as tarballs are
    automatically handled by Buildroot. However, if the package uses a
    non-standard archive format, such as a ZIP or RAR file, or has a
    tarball with a non-standard organization, this variable allows to
    override the package infrastructure default behavior.
  
  * +LIBFOO_CONFIGURE_CMDS+ lists the actions to be performed to
    configure the package before its compilation.
  
  * +LIBFOO_BUILD_CMDS+ lists the actions to be performed to
    compile the package.
  
  * +HOST_LIBFOO_INSTALL_CMDS+ lists the actions to be performed
    to install the package, when the package is a host package. The
    package must install its files to the directory given by
    +$(HOST_DIR)+. All files, including development files such as
    headers should be installed, since other packages might be compiled
    on top of this package.
  
  * +LIBFOO_INSTALL_TARGET_CMDS+ lists the actions to be
    performed to install the package to the target directory, when the
    package is a target package. The package must install its files to
    the directory given by +$(TARGET_DIR)+. Only the files required for
    'execution' of the package have to be
    installed. Header files, static libraries and documentation will be
    removed again when the target filesystem is finalized.
  
  * +LIBFOO_INSTALL_STAGING_CMDS+ lists the actions to be
    performed to install the package to the staging directory, when the
    package is a target package. The package must install its files to
    the directory given by +$(STAGING_DIR)+. All development files
    should be installed, since they might be needed to compile other
    packages.
  
  * +LIBFOO_INSTALL_IMAGES_CMDS+ lists the actions to be performed to
    install the package to the images directory, when the package is a
    target package. The package must install its files to the directory
    given by +$(BINARIES_DIR)+. Only files that are binary images (aka
    images) that do not belong in the +TARGET_DIR+ but are necessary
    for booting the board should be placed here. For example, a package
    should utilize this step if it has binaries which would be similar
    to the kernel image, bootloader or root filesystem images.
  
  * +LIBFOO_INSTALL_INIT_SYSV+ and +LIBFOO_INSTALL_INIT_SYSTEMD+ list the
    actions to install init scripts either for the systemV-like init systems
    (busybox, sysvinit, etc.) or for the systemd units. These commands
    will be run only when the relevant init system is installed (i.e. if
    systemd is selected as the init system in the configuration, only
    +LIBFOO_INSTALL_INIT_SYSTEMD+ will be run).
  
  * +LIBFOO_HELP_CMDS+ lists the actions to print the package help, which
    is included to the main +make help+ output. These commands can print
    anything in any format.
    This is seldom used, as packages rarely have custom rules. *Do not use
    this variable*, unless you really know that you need to print help.
  
  The preferred way to define these variables is:
  
  ----------------------
  define LIBFOO_CONFIGURE_CMDS
  	action 1
  	action 2
  	action 3
  endef
  ----------------------
  
  In the action definitions, you can use the following variables:
  
  * +$(FOO_PKGDIR)+ contains the path to the directory containing the
    +foo.mk+ and +Config.in+ files. This variable is useful when it is
    necessary to install a file bundled in Buildroot, like a runtime
    configuration file, a splashscreen image...
  
  * +$(@D)+, which contains the directory in which the package source
    code has been uncompressed.
  
  * +$(TARGET_CC)+, +$(TARGET_LD)+, etc. to get the target
    cross-compilation utilities
  
  * +$(TARGET_CROSS)+ to get the cross-compilation toolchain prefix
  
  * Of course the +$(HOST_DIR)+, +$(STAGING_DIR)+ and +$(TARGET_DIR)+
    variables to install the packages properly.
  
  Finally, you can also use hooks. See xref:hooks[] for more information.