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: Ied5d0bafbd1d01fc1f109c15c4283de7029903c9
Signed-off-by: Colin Cross <ccross@android.com>
Git-commit: 9bb8731d87eba59fe7f0e504f11f5dc995ae1166
Git-repo: https://android.googlesource.com/kernel/common


[jshriram@codeaurora.org: Fixed minor merge conflicts]
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>