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.

Change-Id: I0dc3935e57f01b517aa64eda0c27b0101e9ea3b2
Signed-off-by: Alex Ray <aray@google.com>
Git-commit: e8cec2c1f77a8a90f701fd2cebdea4feba7640bf
Git-repo: https://android.googlesource.com/kernel/common.git


[imaund@codeaurora.org: Resolve trivial merge conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>