kconfig: suppress "configuration written to .config" for syncconfig

The top-level Makefile invokes "make syncconfig" when necessary.
Then, Kconfig displays the following message when .config is updated.

  #
  # configuration written to .config
  #

It is distracting because "make syncconfig" happens during the build
stage, and does nothing important in most cases.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/kconfig/conf.c

@@ -508,6 +508,11 @@ int main(int ac, char **av)
 		input_mode = (enum input_mode)opt;
 		switch (opt) {
 		case syncconfig:
+			/*
+			 * syncconfig is invoked during the build stage.
+			 * Suppress distracting "configuration written to ..."
+			 */
+			conf_set_message_callback(NULL);
 			sync_kconfig = 1;
 			break;
 		case defconfig: