kbuild: always run 'make silentoldconfig' when tree is cleaned

If the file .kconfig.d is missing then make sure to run
'make silentoldconfig', since we have no way to detect if
a Kconfig file has been updated.

-kconfig.d is created by kconfig and is removed as part
of 'make clean' so the situation is likely to occur in reality.

Jan Beulich <JBeulich@novell.com> reported this bug.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/scripts/kconfig/util.c b/scripts/kconfig/util.c
index 1fa4c0b..a711007 100644
--- a/scripts/kconfig/util.c
+++ b/scripts/kconfig/util.c
@@ -33,7 +33,7 @@
 	FILE *out;
 
 	if (!name)
-		name = ".config.cmd";
+		name = ".kconfig.d";
 	out = fopen("..config.tmp", "w");
 	if (!out)
 		return 1;