Merge branch '2022-08-20-enforce-DM_ETH-migration'

Enforce requiring DM_ETH to be enabled for ethernet drivers, as the
migration deadline has well passed. To facilitate this, we remove some
non-migrated platforms and disable networking on a few others. Finally
we remove some of the now-useless non-DM_ETH code in some platforms as a
prerequisite for DM_ETH being set.