Revert "ramips: disable ZyXel Keenetic by default"

This reverts commit c38074de92.

Since ZyXEL Keenetic has actually 8 MiB flash as fixed in the
previous patch, we can re-enable it.

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

target/linux/ramips/image/rt305x.mk

@@ -908,7 +908,6 @@ define Device/kn
   IMAGE_SIZE := 7872k
   DEVICE_TITLE := ZyXEL Keenetic
   DEVICE_PACKAGES := kmod-usb-core kmod-usb2 kmod-usb-ehci kmod-usb-ledtrig-usbport
-  DEFAULT := n
 endef
 TARGET_DEVICES += kn