config.mk: Check for -fstack-usage support

Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Tom Rini <trini@ti.com>

Makefile

@@ -747,7 +747,7 @@ clean:
 	@rm -f $(obj)MLO
 	@rm -f $(TIMESTAMP_FILE) $(VERSION_FILE)
 	@find $(OBJTREE) -type f \
-		\( -name 'core' -o -name '*.bak' -o -name '*~' \
+		\( -name 'core' -o -name '*.bak' -o -name '*~' -o -name '*.su' \
 		-o -name '*.o'	-o -name '*.a' -o -name '*.exe'	\) -print \
 		| xargs rm -f