Custom projects may need vendor specific expansions to the fastboot
command execution. Allow these to be implemented without messing in
the fastboot code directly.
We have a hook for all commands and also one for the "flash" command.
Each hook can decide if the generic command parser is executed
afterwards (return value FASTBOOT_CMD_FALLTHROUGH) or if the generic
parser shall be skipped (return value 0 or negative error code).

This allows board code to implement vendor specific "oem" commands
or to handle the downloaded image in a special way (i.e. do signature
checks on them)

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>