Copy U-Boot to STAGING_DIR_IMAGE (and append it to the EVA-image from
there) to fix image generation using the image-builder.
Also remove the bootloader from DEVICE_PACKAGES and instead use the
BUILD_DEVICES directive from within the U-Boot makefile.
This fixes eva-image generation using the OpenWRT image-builder.
Signed-off-by: David Bauer <mail@david-bauer.net>