This fixes the following problems:
* Add BUILD_DEVICES for legacy subtarget
* Use features from u-boot.mk for sama5 subtarget This is mainly done
by changing the prefix from uboot to U-Boot. This makes them depend
on the sama5 subtarget and not selectable for the legacy subtarget
any more
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>