6LoWPAN: Add configurable support for 6LoWPAN star topology. With this...

6LoWPAN:  Add configurable support for 6LoWPAN star topology.  With this change, the endpoints which are the 'points' of the star will forward all traffic to the coordinator.  The coordinator is assumed to be the 'hub' of the star.  This function also implements IPv6 forwarding.  That forwarding implementation is minimal, just enough to support the forwarding requirements of the star hub node.