kconfig: rename silentoldconfig to syncconfig

As commit cedd55d49dee ("kconfig: Remove silentoldconfig from help
and docs; fix kconfig/conf's help") mentioned, 'silentoldconfig' is a
historical misnomer.  That commit removed it from help and docs since
it is an internal interface.  If so, it should be allowed to rename
it to something more intuitive.  'syncconfig' is the one I came up
with because it updates the .config if necessary, then synchronize
include/generated/autoconf.h and include/config/* with it.

You should not manually invoke 'silentoldcofig'.  Display warning if
used in case existing scripts are doing wrong.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Ulf Magnusson <ulfalizer@gmail.com>
diff --git a/Makefile b/Makefile
index e02d092..5675aa1 100644
--- a/Makefile
+++ b/Makefile
@@ -588,7 +588,7 @@
 # include/generated/ and include/config/. Update them if .config is newer than
 # include/config/auto.conf (which mirrors .config).
 include/config/%.conf: $(KCONFIG_CONFIG) include/config/auto.conf.cmd
-	$(Q)$(MAKE) -f $(srctree)/Makefile silentoldconfig
+	$(Q)$(MAKE) -f $(srctree)/Makefile syncconfig
 else
 # external modules needs include/generated/autoconf.h and include/config/auto.conf
 # but do not care if they are up-to-date. Use auto.conf to trigger the test