ANDROID: arm64: add option to build Image-dtb

Some bootloaders couldn't decompress Image.gz-dtb.

Change-Id: I698cd0c4ee6894e8d0655d88f3ecf4826c28a645
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>

[AmitP: Folded following android-4.9 commit changes into this patch
        56b70ac2447f ("ANDROID: ARM64: Ignore Image-dtb from git point of view")]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>