[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.
It will also be called for every sent IP packet, as the destination address is not necessarily the address of the next hop. 

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?