gluon-core: primary-mac: replace all eth0/eth1 lookups with lan/wan

The netdev() lookup is confusing to use: whenever a interface does not
exist during boot (for example VLAN) or when the address is overridden
from board.json (which is not obvious at all), it will yield either no
address, or a different address than expected.

To avoid this confusion, using board.json-based interface() is
preferable. This converts all uses of netdev() to the corresponding
lan/wan lookups, except for the final fallback for eth0.