ramips: disable ZyXel Keenetic by default

Disable the ZyXel Keenetic images by default as the device has
insufficient flash space for release build images.

Ref: https://forum.openwrt.org/t/devices-too-big-to-save-overlay/18161/72
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/ramips/image/rt305x.mk

@@ -908,6 +908,7 @@ define Device/kn
   IMAGE_SIZE := $(ralink_default_fw_size_4M)
   DEVICE_TITLE := ZyXEL Keenetic
   DEVICE_PACKAGES := kmod-usb-core kmod-usb2 kmod-usb-ehci kmod-usb-ledtrig-usbport
+  DEFAULT := n
 endef
 TARGET_DEVICES += kn