U-Boot machine/arch independent API for external apps
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1. Main assumptions

- there is a single entry point (syscall) to the API

- per current design the syscall is a C-callable function in the U-Boot
text, which might evolve into a real syscall using machine exception trap
once this initial version proves functional

- the consumer app is responsible for producing appropriate context (call
number and arguments)

- upon entry, the syscall dispatches the call to other (existing) U-Boot
functional areas like networking or storage operations

- consumer application will recognize the API is available by searching
a specified (assumed by convention) range of address space for the
signature

- the U-Boot integral part of the API is meant to be thin and non-intrusive,
leaving as much processing as possible on the consumer application side,
for example it doesn't keep states, but relies on hints from the app and
so on

- optional (CONFIG_API)

2. Calls

- console related (getc, putc, tstc etc.)
- system (reset, platform info)
- time (delay, current)
- env vars (enumerate all, get, set)
- devices (enumerate all, open, close, read, write); currently two classes
of devices are recognized and supported: network and storage (ide, scsi,
usb etc.)

3. Structure overview

- core API, integral part of U-Boot, mandatory
- implements the single entry point (mimics UNIX syscall)

- glue
- entry point at the consumer side, allows to make syscall, mandatory
part

- helper conveniency wrappers so that consumer app does not have to use
the syscall directly, but in a more friendly manner (a la libc calls),
optional part

- consumer application
- calls directly, or leverages the provided glue mid-layer