ARM: build correct dtbs to append to zImage

The boot/dts/Makefile and boot/dts/*/Makefile provide DTB_LIST which is
equal to dtb-y when CONFIG_BUILD_ARM_APPENDED_DTB_IMAGE=y.
Include these Makefiles to build dtbs for dtb appended zImage.

Change-Id: I5182d88909e68644905a6c91c2cef14642415417
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>