kconfig: handle format string before calling conf_message_callback()

As you see in mconf.c and nconf.c, conf_message_callback() hooks are
likely to end up with the boilerplate of vsnprintf().  Process the
string format before calling conf_message_callback() so that it
receives a simple string.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Dirk Gouders <dirk@gouders.net>
diff --git a/scripts/kconfig/nconf.c b/scripts/kconfig/nconf.c
index 97b7844..5cbdb92 100644
--- a/scripts/kconfig/nconf.c
+++ b/scripts/kconfig/nconf.c
@@ -1210,12 +1210,9 @@ static void conf(struct menu *menu)
 	}
 }
 
-static void conf_message_callback(const char *fmt, va_list ap)
+static void conf_message_callback(const char *s)
 {
-	char buf[1024];
-
-	vsnprintf(buf, sizeof(buf), fmt, ap);
-	btn_dialog(main_window, buf, 1, "<OK>");
+	btn_dialog(main_window, s, 1, "<OK>");
 }
 
 static void show_help(struct menu *menu)