build: make NETGEAR_REGION optional in netgear-chk

This patch makes specifying NETGEAR_REGION optional, in which case
mkchkimage will default to region 1 (WW).

Signed-off-by: Joseph C. Lehner <joseph.c.lehner@gmail.com>

include/image-commands.mk

@@ -49,7 +49,7 @@ define Build/netgear-chk
 		-o $@.new \
 		-k $@ \
 		-b $(NETGEAR_BOARD_ID) \
-		-r $(NETGEAR_REGION)
+		$(if $(NETGEAR_REGION),-r $(NETGEAR_REGION),)
 	mv $@.new $@
 endef