image-commands.mk: Use ERROR_MESSAGE for imagesize fails

If a image is bigger than the device can handle, an error message is
printed. This is usually silenced and silently ignored, making it harder
to debug. While it's possible to run the build in verbose mode (via
`make V=s`) and grep for *is too big*, it's more intuitive to print the
error message directly. For that use the newly unlocked `$(call
ERROR_MESSAGE,...)` definition which now also print in non-verbose mode.

Fixes: FS#50 (aka openwrt#7604)

Signed-off-by: Paul Spooren <mail@aparcar.org>

include/image-commands.mk

@@ -191,7 +191,7 @@ define Build/check-size
 	@imagesize="$$(stat -c%s $@)"; \
 	limitsize="$$(($(subst k,* 1024,$(subst m, * 1024k,$(if $(1),$(1),$(IMAGE_SIZE))))))"; \
 	[ $$limitsize -ge $$imagesize ] || { \
-		echo "WARNING: Image file $@ is too big: $$imagesize > $$limitsize" >&2; \
+		$(call ERROR_MESSAGE,    WARNING: Image file $@ is too big: $$imagesize > $$limitsize); \
 		rm -f $@; \
 	}
 endef