build: dt_image: support prebuilt DT images

If for whatever reason we need a prebuilt DT image,
it has to end up in target files for the signing slave.
Change BOARD_KERNEL_PREBUILT_DT to hold the path of the
prebuilt DT image, if it's going to be used.

Change-Id: I8042d347d271da5c3963824777d399bbc4229aa9