With CONFIG_BOOTM_FORCE_SIGNED_IMAGES the bootm code will refuse to boot
unsigned images. Since currently FIT is the only image type which
supports signing, this means with this option we enforce using FIT
images. All additionally passed in device trees and initrds will be
ignored, so that only the ones from the FIT image can be used.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>