ARM: Fix dtb list when DTB_IMAGE_NAMES is empty

In the 3.10 kernel, dtb-y is not defined in Makefile.boot anymore
but in dts/Makefile, so it needs to be included too.

Change-Id: I6d6fccf933709bcb6220ce8f12b4b9e2a7c40d63
Signed-off-by: Benoit Goby <benoit@android.com>