Allow for the DTB image to be built by the Android build system.

The current solution expects BOARD_PREBUILT_DTBIMAGE_DIR to
contain prebuilt DTB files that are concatenated by the build system
to create $OUT/dtb.img. In order to accommodate devices that build
the dtb image locally, make boot.img creation depend on $OUT/dtb.img
only when BOARD_PREBUILT_DTBIMAGE_DIR is undefined.

Bug: 133161451
Test: Build with BOARD_PREBUILT_DTBIMAGE_DIR undefined and verify
using unpack_bootimg.py that $OUT/dtb.img was included in boot.img.

Change-Id: Iae2c634ccdc1d83589b26d382882f75fb8565a31