update lan, wan separation for o2 6431

Co-authored-by: Jan-Niklas Burfeind <github@aiyionpri.me>
removed x86-legacy, added ramips-mt7621

The packages necessary to get the DSL modem working increase the
squashfs size by around 1MB.

Remove them from Gluon, as this functionality is not supported.

There is no factory image, they are flashed using the sysupgrade image.

Flash instructions in the OpenWRT Commit: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=eae6cac6a30b95bef04d033ed8a303b7e35a078b