kbuild: add %.dtb.S and %.dtb to 'targets' automatically

Another common pattern that consists of chained commands is to compile
a DTB as binary data into the kernel image or a module.  It is used in
several places in the source tree.  Support it in the core Makefile.

$(call if_changed,dt_S_dtb) is more suitable than $(call cmd,dt_S_dtb)
in case cmd_dt_S_dtb is changed in the future.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Frank Rowand <frowand.list@gmail.com>

scripts/Makefile.lib

@@ -279,8 +279,8 @@ cmd_dt_S_dtb=						\
 	echo '.balign STRUCT_ALIGNMENT'; 		\
 ) > $@
 
-$(obj)/%.dtb.S: $(obj)/%.dtb
-	$(call cmd,dt_S_dtb)
+$(obj)/%.dtb.S: $(obj)/%.dtb FORCE
+	$(call if_changed,dt_S_dtb)
 
 quiet_cmd_dtc = DTC     $@
 cmd_dtc = mkdir -p $(dir ${dtc-tmp}) ; \