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 
(for the record, I only introduced this change after I realized this flaw in `rpl_udp`).