kernel-build.mk: add support for compiling only DTS

Add support for compiling DTS for the selected target. This can be
useful for testing if the DTS correctly compile and doesn't produce any
error.

This adds a new make target. To compile only DTS use:

make target/linux/dtb

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit c47532b1ea)

include/kernel-build.mk

@@ -156,6 +156,10 @@ define BuildKernel
   compile: $(LINUX_DIR)/.modules
 	$(MAKE) -C image compile TARGET_BUILD=
 
+  dtb: $(STAMP_CONFIGURED)
+	$(_SINGLE)$(KERNEL_MAKE) scripts_dtc
+	$(MAKE) -C image compile-dtb TARGET_BUILD=
+
   oldconfig menuconfig nconfig xconfig: $(STAMP_PREPARED) $(STAMP_CHECKED) FORCE
 	rm -f $(LINUX_DIR)/.config.prev
 	rm -f $(STAMP_CONFIGURED)