ipq40xx: check kernel-size for NBG6617

The ZyXEL NBG6617 has a separate kernel partition which is 4MiB large.

Add the kernel size to validate the kernel won't be bigger than this
fixed limit.

Signed-off-by: David Bauer <[email protected]>

target/linux/ipq40xx/image/Makefile

@@ -661,6 +661,7 @@ define Device/zyxel_nbg6617
 	DEVICE_VENDOR := ZyXEL
 	DEVICE_MODEL := NBG6617
 	SOC := qcom-ipq4018
+	KERNEL_SIZE := 4096k
 	ROOTFS_SIZE := 24960k
 	RAS_BOARD := NBG6617
 	RAS_ROOTFS_SIZE := 19840k