only update version header as needed

Constantly rebuilding the version header will force useless relinking, so we
simply need to compare the new header with the existing one before updating
it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

Makefile

@@ -343,7 +343,8 @@ $(VERSION_FILE):
 		echo -n "$(U_BOOT_VERSION)" ; \
 		echo -n $(shell $(CONFIG_SHELL) $(TOPDIR)/tools/setlocalversion \
 			 $(TOPDIR)) ; \
-		echo "\"" ) > $(VERSION_FILE)
+		echo "\"" ) > $@.tmp
+		@cmp -s $@ $@.tmp && rm -f $@.tmp || mv -f $@.tmp $@
 
 gdbtools:
 		$(MAKE) -C tools/gdb all || exit 1