lantiq: enlarge ZyXEL P-2812HNU-F1 kernel partition (FS#2124)

lantiq: enlarge ZyXEL P-2812HNU-F1 kernel partition (FS#2124)

The current snapshot release kernel with it's 2119245 Byte
size is too big to fit into the 2097152 (2MiB) area that
was set aside for the kernel. Which causes the device to
fail to boot (after an update or even during a fresh install)

 NAND read: device 0 offset 0x60000, size 0x200000
  2097152 bytes read: OK
 ## Booting kernel from Legacy Image at 80800000 ...
    Image Name:   MIPS OpenWrt Linux-4.14.98
    Created:      2019-02-13   9:37:36 UTC
    Image Type:   MIPS Linux Kernel Image (lzma compressed)
    Data Size:    2119245 Bytes = 2 MiB
    Load Address: 80002000
    Entry Point:  80002000
    Verifying Checksum ... Bad Data CRC
 ERROR: can't get kernel image!

This patch fixes the problem by enlarging the kernel partition
at the cost of the ubi/data partition behind it. The patch
also adds a KERNEL_SIZE variable to the image Makefile to
prevent silent corruptions from happening in the future.

Please note: The u-boot environment for the router also
needs to be updated. So please attach an appropriate
serial converter cable and enter the following commands
into the u-boot prompt:

 setenv nboot 'nand read 0x80800000 0x60000 0x300000; bootm 0x80800000'
 saveenv

The wiki has been updated accordingly:
<https://openwrt.org/toh/zyxel/p2812hnu-f1>

Fixes: FS#2124
Suggested-by: Mafketel (User on bugs.openwrt.org)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>