add lldp support

[isomer]

It would greatly easy debugging if all network elements implement lldp. We should be the change we want to see in the world and make sure erbium supports lldp.

phase 1: support sending lldp information on configured ports (default: all ethernet interfaces?)

phase 2: parse lldp information received on all interfaces and present it via /metrics and /api/v1/lldp.json.

extra points awarded for supporting lldp-med (both rx and tx).