lantiq: set maximum kernel size for P2812HNUF3

The board has a fixed size kernel partition but do not limit the kernel
size during image building.

Disable image building for both boards as well, since the kernel of the
last release as well as master are to big to fit into the 2 MByte kernel
partition.

Signed-off-by: Mathias Kresin <dev@kresin.me>

target/linux/lantiq/image/vr9.mk

@@ -282,6 +282,8 @@ define Device/zyxel_p-2812hnu-f3
   DEVICE_VARIANT := F3
   BOARD_NAME := P2812HNUF3
   DEVICE_PACKAGES := kmod-rt2800-pci wpad-basic-wolfssl kmod-usb-dwc2
+  KERNEL_SIZE := 2048k
   SUPPORTED_DEVICES += P2812HNUF3
+  DEFAULT := n
 endef
 TARGET_DEVICES += zyxel_p-2812hnu-f3