Support for RTL838x based managed switches

@bmork kindly run me by the exact sequence to make it work please. nbd added that new bridge-vlan feature to /e/c/network and it is not working out of the box for me. I have /etc/board.d patches ready to make the unit come up by default with a master switch0 bridge using that API. however the moment i add a vlan traffic dead locks. If I know the manual steps to fix it, I can ask Felix to augment that away inside netifd.

I was also thinking that it might make sense to have the default setup, that the switches run dhcp and have a static ip on vlan 100 on port1 for mgmt purposes ?!