kernel: Allow overriding dtb target

On some devices we might want to build just the *.dtb files
that we actually need instead of the every dtb that exists.

Change-Id: Id7a152499d4ee1dd08f79d34cc34bfe959313515
diff --git a/build/tasks/kernel.mk b/build/tasks/kernel.mk
index 5ea7962..72c32f9 100644
--- a/build/tasks/kernel.mk
+++ b/build/tasks/kernel.mk
@@ -481,7 +481,7 @@
 	@echo "Building dtb.img"
 	$(hide) find $(DTB_OUT)/arch/$(KERNEL_ARCH)/boot/dts -type f -name "*.dtb" | xargs rm -f
 	$(call make-dtb-target,$(KERNEL_DEFCONFIG))
-	$(call make-dtb-target,dtbs)
+	$(call make-dtb-target,$(TARGET_KERNEL_DTB))
 	cat $(shell find $(DTB_OUT)/arch/$(KERNEL_ARCH)/boot/dts -type f -name "*.dtb" | sort) > $@
 	$(hide) touch -c $(DTB_OUT)
 endif # !BOARD_PREBUILT_DTBIMAGE_DIR