Commands are added to U-Boot by creating a new command structure.
  This is done by first including command.h, then using the U_BOOT_CMD() macro
  to fill in a cmd_tbl_t struct.
  
  U_BOOT_CMD(name,maxargs,repeatable,command,"usage","help")
  
  name:	 is the name of the commad. THIS IS NOT a string.
  maxargs: the maximum number of arguments this function takes
  repeatable: either 0 or 1 to indicate if autorepeat is allowed
  command: Function pointer (*cmd)(struct cmd_tbl_s *, int, int, char *[]);
  usage:	 Short description. This is a string
  help:	 Long description. This is a string
  
  
  **** Behind the scene ******
  
  The structure created is named with a special prefix and placed by
  the linker in a special section using the linker lists mechanism
  (see include/linker_lists.h)
  
  This makes it possible for the final link to extract all commands
  compiled into any object code and construct a static array so the
  command array can be iterated over using the linker lists macros.
  
  The linker lists feature ensures that the linker does not discard
  these symbols when linking full U-Boot even though they are not
  referenced in the source code as such.
  
  If a new board is defined do not forget to define the command section
  by writing in u-boot.lds ($(srctree)/board/boardname/u-boot.lds) these
  3 lines:
  
  	.u_boot_list : {
  		KEEP(*(SORT(.u_boot_list*)));
  	}