kernel: Move our dtb.img logic here

Our build/make changes break incremenetal builds with prebuilt dtb dir
(see discussion here: https://android-review.googlesource.com/c/platform/build/+/1134337/1/core/Makefile )

Instead of using the AOSP recipe, just define our own and use it by not defining
BOARD_PREBUILT_DTBIMAGE_DIR

Change-Id: If579b1670db447557b6f76ca79ecec22661c4080