ANDROID: ARM64: add option to build Image.gz/dtb combo

Allows a defconfig to set a list of dtbs to concatenate with an
Image.gz to create a Image.gz-dtb.

Includes 8adb162 arm64: Fix correct dtb clean-files location

Change-Id: I0b462322d5c970f1fdf37baffece7ad058099f4a
Signed-off-by: Alex Ray <aray@google.com>
[rishabhb] Use subdir to descend into device tree directories
Signed-off-by: Rishabh Bhatnagar <rishabhb@codeaurora.org>