sys/net: Add NHDP protocol implementation

This PR introduces the MANET Neighborhood Discovery Protocol specified in [RFC 6130](https://tools.ietf.org/html/rfc6130). The implementation is based on oonf_api and utlist. It introduces a single thread for protocol message sending and receiving.

It is still work in progress in some parts and needs thoroughly testing. TODOs include:
- [ ] Cleanup timed out information base entries
- [ ] Optimize list loops
- [ ] Provide a simple example application
- [ ] Add proper documentation
I think @OlegHahm suggests to use a conditional definition (compare https://github.com/RIOT-OS/RIOT/blob/master/sys/net/routing/etx_beaconing.c#L40).