flash_{un,}lock: support both lock/unlock in the same binary

flash_{un,}lock: support both lock/unlock in the same binary

Add new --lock/--unlock flags, so we can do either with the same binary.
This will prepare for the addition of other features, so we don't have
to keep duplicating the same binary via #include "flash_unlock.c".

The defaults still work as expected: flash_unlock will default to
REQUEST_UNLOCK, and flash_lock will default to REQUEST_LOCK.

Eventually, we might deprecate one of the two (flash_unlock, probably),
so we only have to ship one flash_{un,}lock binary.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>