ARM: convert build of appended dtb zImage to list of dtbs

Allow CONFIG_BUILD_ARM_APPENDED_DTB_IMAGE_NAMES to specify
a space separated list of dtbs to append to the zImage,
and name the resulting file zImage-dtb

Change-Id: I36d9108a2349bdbb373e95076dcb1417d8c7dce6
Signed-off-by: Colin Cross <ccross@android.com>

Conflicts:
	arch/arm/boot/Makefile
	scripts/Makefile.lib