sixlowpan: make neighbor discovery work for now

This is only a hack to have Neighbor discovery work for the next release. It's not pretty, it uses a hullovalot memory, but I hope it produces no dead locks and it floods the network for a little while.
**Upon first send to an unknown node there might be an error notification** on ipv6_send_packet(), but the packet is send regardless.

Depends on #1853 
No, if there is no routing protocol registered (== `ip_get_next_hop == NULL`, and a valid case to send packets anyway via information of the ND) this would always fail if *dest* would not be set. Nice example (though I don't know exactly why this is): in the `rpl_udp` application `init r` does not set the routing provider. As a result, no data would be sendable from the router, since it always lands in the removed if branch.