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 
Maybe I misunderstand this. What is the solicited IP address set to here? Is it all_nodes multicast? In this case it is in contradiction with RFC 4861 (see page 22, about the field "Target Address"). If it is not all_nodes, then how do we bootstrap to know the IP address of the destination in the first place? One way to do that would be to send pseudo-periodic (period+ small jitter) unsolicited neighbor advertisements, as described in section 7.2.6. of RFC 4861.