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
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index a654bea..a1b35e4 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -305,10 +305,11 @@
 dtboimage: $(INSTALLED_DTBOIMAGE_TARGET)
 
 ifeq ($(BOARD_INCLUDE_DTB_IN_BOOTIMG),true)
-$(BOARD_PREBUILT_DTBIMAGE_DIR):
+$(INSTALLED_DTBIMAGE_TARGET):
 	echo -e ${CL_GRN}"Building DTBs"${CL_RST}
 	$(call make-dtb-target,$(KERNEL_DEFCONFIG))
 	$(call make-dtb-target,dtbs)
+	cat $(shell find $(DTBS_OUT)/arch/$(KERNEL_ARCH)/boot/dts/** -type f -name "*.dtb" | sort) > $@
 .PHONY: dtbimage
 dtbimage: $(INSTALLED_DTBIMAGE_TARGET)
 endif # BOARD_INCLUDE_DTB_IN_BOOTIMG