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 
The definition of the solicited-nodes multicast address is in RFC 4291, section 2.7.1:

>       Solicited-Node Address:  FF02:0:0:0:0:1:FFXX:XXXX
>
>   Solicited-Node multicast address are computed as a function of a
>   node's unicast and anycast addresses.  A Solicited-Node multicast
>   address is formed by taking the low-order 24 bits of an address
>   (unicast or anycast) and appending those bits to the prefix
>   FF02:0:0:0:0:1:FF00::/104 resulting in a multicast address in the
>   range

The corresponding address is registered to an interface on [6lowpan interface initialization](https://github.com/RIOT-OS/RIOT/blob/master/sys/net/network_layer/sixlowpan/lowpan.c#L1739) so a node is always reachable under this address so no bootstrapping is required. Periodic neighbor advertisements would require a new thread and as discussed below this is not really an option + it would flood the network even more, where this PR only would flood it if a link-layer address is currently unknown.