fastboot: Implement generic fastboot_set_reboot_flag

It is possible to implement fastboot_set_reboot_flag in a generic way
if BCB commands are turned on for a target. Using
bcb_set_reboot_reason allows to do this by simply passing string with
correct reboot reason that should be handled during next boot process.

If BCB are turned off, then bcb_set_reboot_reason would simply return
error, so it won't introduce any new behaviour for such targets.

Signed-off-by: Roman Kovalivskyi <roman.kovalivskyi@globallogic.com>