[RFC] add olsr2 routing algorithm 

This adds an implementation of the OLSRv2 routing protocol (without metrics), based on oonf_api.
The interface offered to applications currently consists of only a `olsr_init` function which will set up and start a sender and receiver thread for RFC5444 based (UDP on port 269) HELLO and Topology Control messages.
That means that every `HELLO_REFRESH_INTERVAL` (2s + random jitter) a non-forwarded HELLO message is send for neighborhood discovery. Every `TC_REFRESH_INTERVAL` (5s + random jitter) a topology control message is broadcast (only by selected flooding MPRs) through the network.

This happens transparent to the application, IP packets are now routed to known hosts within the MANET once a path has been established. 

`print_topology_set` will output the forwarding table with all known mesh nodes.

Does work well on native with #680, but still crashes on the testbed after some time.
Each known MANET router requires 72 byte of memory + 12 bytes for each alternative route to the node.

Depends on #630
Again: use _head != NULL instead?