ANDROID: ARM: add config option to build zImage/dtb combo

Allows a defconfig to set a default dtb to concatenate with a zImage
to create a zImage-dtb.<dtb name>.

Change-Id: I34b643b1c49228fbae88a56e46c93c478089620d
Signed-off-by: Erik Gilling <konkers@android.com>
Git-commit: 23a70d4e7b8864cf405f997d3608de65a82cf87d
Git-repo: https://android.googlesource.com/kernel/common


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