gluon-core: add unique node_id

This adds a `node_id` to announce.d. `node_id` is an hex-number uniquely
identifying a particular node. On nodes having a
primary MAC this MAC address will be used as `node_id` (sans the
colons). The value is opaque. It must not be converted back into
a MAC address.

`node_id` should be sent in all alfred and announced packets to
ensure clients can link the data to a particular node.
Merge pull request #42 from freifunk-gluon/node-id

gluon-core: add unique node_id