make flush, should always be visible

Makefile

@@ -473,11 +473,12 @@ clean:
 distclean: clean
 	rm -rf sources/*
 
-flush:
-	rm -f $(PROJECT_BUILD_DIR)/tgt-config.cache
 
 endif # ifeq ($(BR2_HAVE_DOT_CONFIG),y)
 
+flush:
+	rm -f $(PROJECT_BUILD_DIR)/tgt-config.cache
+
 %_defconfig: $(CONFIG)/conf
 	cp $(shell find ./target/ -name $@) .config
 	-@$(MAKE) oldconfig