include/toplevel.mk: fix defconfig when ~/.openwrt/defconfig exists

./scripts/feeds update will reset the .config file if ~/.openwrt/defconfig
exists, thus resetting the target to ar71xx.

Signed-off-by: John Crispin <john@phrozen.org>

include/toplevel.mk

@@ -110,7 +110,7 @@ config-clean: FORCE
 
 defconfig: scripts/config/conf prepare-tmpinfo FORCE
 	touch .config
-	@if [ -e $(HOME)/.openwrt/defconfig ]; then cp $(HOME)/.openwrt/defconfig .config; fi
+	@if [  -s .config -a -e $(HOME)/.openwrt/defconfig ]; then cp $(HOME)/.openwrt/defconfig .config; fi
 	$< --defconfig=.config Config.in
 
 confdefault-y=allyes