base-files: emit tagged switch configuration by default

Instead of only using tagged CPU port configurations when more than one VLAN
is present on the switch, always emit tagged configurations unless a board
explicitely opts out of this behaviour by using the previously introduced
[0-9]u@netdev syntax.

Emitting default tagged configurations has the following benefits:

 - Relation of switch vlans to netdevs is easier to understand, especially
   for multi-cpu-port switches

 - Adding additional VLANs (e.g. to break out a LAN port for other purposes)
   becomes easier as users are not forced to change the existing untagged
   VLAN to tagged and the existing ifname notation from ethX to ethX.Y
   anymore, drastly reducing the likelyhood of soft-bricks.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>