arm64: Properly compile Image.gz-dtb

Image.gz-dtb has a dependency on Image.gz. Another Image.gz in the
makefile will trigger generating the Image.gz as well.
In a multi-thread build, one of the thread could potentially start
generating Image.gz-dtb in parallel, and find Image.gz in the
filesystem but in an incomplete state since it is still being
generated by the other thread.
Appending dtb to the incomplete Image.gz will create a bad Image.gz-dtb.

Change-Id: Ie8c21065fcdce9384827e0b5844173373319106c
Signed-off-by: Runmin Wang <runminw@codeaurora.org>