Enhancements to the IPv6 Stack

These commits add some features to the BLIP/RPL stack.

1. Neighbor Discovery. BLIP nodes can now send and receive router solicititations and router advertisement messages. This allows them to autoconfigure their IPv6 addresses.
2. RPL updated. Parts of RPL were not up to spec and not compatible with Contiki. This fixes at least some of those issues.
3. RPL prefix information. If a network is running RPL, prefix information for address autoconfiguration can be disseminated with the DIO messages. This adds that feature.

This PR should make it easier to deploy IPv6 nodes that can be moved to a new network without having to be reprogrammed (because of the new addressing modes). I've done some very little testing (mostly compatibility with Contiki) so I'm not ready to merge these commits into master, but I wanted to put them here to get the process moving.