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>
                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: