ath79: Add missing IMAGE_SIZE for Etactica EG200

The Etactica EG200 is the only device in ath79 despite nand
target that lacks IMAGE_SIZE.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/ath79/image/generic.mk

@@ -392,6 +392,7 @@ define Device/etactica_eg200
   DEVICE_MODEL := EG200
   DEVICE_PACKAGES := kmod-usb-chipidea2 kmod-ledtrig-oneshot \
 	kmod-usb-serial kmod-usb-serial-ftdi kmod-usb-storage  kmod-fs-ext4
+  IMAGE_SIZE := 16000k
   SUPPORTED_DEVICES += rme-eg200
 endef
 TARGET_DEVICES += etactica_eg200