kernel: Build dtb.img from compiled dtbs

Change-Id: If419d090740c005357fbd1665f78296d106b4509
diff --git a/config/BoardConfigKernel.mk b/config/BoardConfigKernel.mk
index b0a75ef..580baa0 100644
--- a/config/BoardConfigKernel.mk
+++ b/config/BoardConfigKernel.mk
@@ -137,6 +137,10 @@
 BOARD_PREBUILT_DTBOIMAGE ?= $(PRODUCT_OUT)/dtbo-pre.img
 endif
 
+ifeq ($(BOARD_INCLUDE_DTB_IN_BOOTIMG),true)
+BOARD_PREBUILT_DTBIMAGE_DIR ?= $(PRODUCT_OUT)/dtbs/arch/$(KERNEL_ARCH)/boot/dts/**
+endif
+
 # Set use the full path to the make command
 KERNEL_MAKE_CMD := $(BUILD_TOP)/prebuilts/build-tools/$(HOST_OS)-x86/bin/make