kernel: Allow to limit the used .dtb files for the final DTB image

On some devices too many .dtb files are generated. Instead of taking
all of them, allow to only take selected .dtb files for the final
DTB image.

Example: Motorola Edge 40 Pro (rtwo, sm8550) generates the following:
obj/DTB_OBJ/out/kaka-rumi.dtb
obj/DTB_OBJ/out/kalama-rtwo-base-kalama-rtwo-evb1-overlay.dtb
obj/DTB_OBJ/out/kalama-rtwo-base-kalama-rtwo-evt1-overlay.dtb
obj/DTB_OBJ/out/kalama-rtwo-base-kalama-rtwo-na-vzw-evt2-overlay.dtb
obj/DTB_OBJ/out/kalama-rtwo-base-kalama-rtwo-prc-dvt1b-overlay.dtb
obj/DTB_OBJ/out/kalama-rtwo-base-kalama-rtwo-prc-pvt-overlay.dtb
obj/DTB_OBJ/out/kalama-rtwo-base-kalama-rtwo-row-dvt1b-overlay.dtb
obj/DTB_OBJ/out/kalama-rtwo-base.dtb
obj/DTB_OBJ/out/kalama-v2-rtwo-base-kalama-rtwo-evb1-overlay.dtb
obj/DTB_OBJ/out/kalama-v2-rtwo-base-kalama-rtwo-evt1-overlay.dtb
obj/DTB_OBJ/out/kalama-v2-rtwo-base-kalama-rtwo-na-vzw-evt2-overlay.dtb
obj/DTB_OBJ/out/kalama-v2-rtwo-base-kalama-rtwo-prc-dvt1b-overlay.dtb
obj/DTB_OBJ/out/kalama-v2-rtwo-base-kalama-rtwo-prc-pvt-overlay.dtb
obj/DTB_OBJ/out/kalama-v2-rtwo-base-kalama-rtwo-row-dvt1b-overlay.dtb
obj/DTB_OBJ/out/kalama-v2-rtwo-base.dtb

with only kalama-rtwo-base.dtb and kalama-v2-rtwo-base.dtb needed.
Disabling compilation of the other kalama.dtb isn't possible, as this
would also disable the compilation of the needed .dtbo files.
Because they are earlier in alphabetical order, and the matching
board ids, the bootloader ends up selecting those, instead of the
correct ones. This results in USB (and possible other things) to
not work.

Change-Id: I486b6ec538a3e4906b5b8aa5ecb355b480b60de7