netifd: rename device section

Rename device section to more user friendly LANX and WAN. This will
appear in Ethernet.Interface.{i}.Alias. The value for the parameter
Ethernet.Interface.{i}.Name will remain the underlying device name
for example, eth0.1, eth1 etc.

Note: for this to take effect, device will need default reset since
the change is in uci-defaults and considering this is a cosmetic change,
migrations over upgrades is not accounted for.