[RFC] allow applications to register a function to provide routing information

Currently, RIOT does not offer a way to provide the IP stack with routing information.
This patch adds a very simple way to do so, a routing protocol implementation would register a function get_next_hop() and each time RIOT receives a packet that is not destined for the local interface, it asks the routing implementation what to do with it.

The routing implementation might hold a lock to protect it's data structures when they are currently modified, so the registered function may block. Is this a problem?