ramips: add IMAGE_SIZE to DEVICE_VARS, it is used by build templates

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

SVN-Revision: 48611

target/linux/ramips/image/Makefile

@@ -22,7 +22,7 @@ define Image/Build/Initramfs
 	$(call Image/Build/Profile/$(PROFILE),initramfs)
 endef
 
-DEVICE_VARS += DTS
+DEVICE_VARS += DTS IMAGE_SIZE
 
 loadaddr-y := 0x80000000
 loadaddr-$(CONFIG_TARGET_ramips_rt288x) := 0x88000000