kbuild: use cmd_shipped instead of cmd_copy

We already have cmd_shipped in scripts/Makefile.lib.
Use it rather than defining a new command cmd_copy.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

dts/Makefile

@@ -14,11 +14,8 @@ endif
 
 DTB := arch/$(ARCH)/dts/$(DEVICE_TREE).dtb
 
-quiet_cmd_copy = COPY    $@
-      cmd_copy = cp $< $@
-
 $(obj)/dt.dtb: $(DTB) FORCE
-	$(call if_changed,copy)
+	$(call if_changed,shipped)
 
 targets += dt.dtb