build: image: set default parameter for check-size

In most cases check-size is used with IMAGE_SIZE and vice versa. Let check-size
use IMAGE_SIZE by default.

Signed-off-by: Sungbo Eo <[email protected]>

include/image-commands.mk

@@ -269,7 +269,7 @@ define Build/xor-image
 endef
 
 define Build/check-size
-	@[ $$(($(subst k,* 1024,$(subst m, * 1024k,$(1))))) -ge "$$(stat -c%s $@)" ] || { \
+	@[ $$(($(subst k,* 1024,$(subst m, * 1024k,$(if $(1),$(1),$(IMAGE_SIZE)))))) -ge "$$(stat -c%s $@)" ] || { \
 		echo "WARNING: Image file $@ is too big" >&2; \
 		rm -f $@; \
 	}