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>