lantiq: grow kernel partition Zyxel P-2812HNU-F1

lantiq: grow kernel partition Zyxel P-2812HNU-F1

Change partition table in dts file.
Change DEVICE_COMPAT_VERSION
Enable automatic build.

To take advantage of the bigger kernel partition,
the uboot environment has to be changed:
setenv nboot 'nand read 0x81000000 0x60000 0x500000; bootm 0x81000000'
setenv bootcmd 'run nboot'
saveenv

Of course you need a u-boot capable of handling this.
The u-boot discussed in this forum thread:
https://forum.openwrt.org/t/zyxel-p2812hnu-f1-u-boot/100281
should be able to handle kernels up to an uncompressed size of 16MiB.

Signed-off-by: Isaac de Wolff <idewolff@gmx.com>
Link: https://github.com/openwrt/openwrt/pull/17209
Link: https://github.com/openwrt/openwrt/pull/17300
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 0d21cc8a92ea3b2cc94efefd60eae968a4296543)