For filtering of Bootloader Spec entries, Bootloader Spec
specify an optional key machine-id. By set the
global.boot.machine-id variable the checking of machine-id
key in Bootloader Spec entries will be activate. If the
variable and key match, appropriate boot entry will be
booting. If it not match boot entry will be ignore and
barebox check the next boot entry.

Signed-off-by: Andreas Schmidt <mail@schmidt-andreas.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>