IPv6 fixes:  Both ICMPv6 autonomous auto-configuration and the ICMPv6 router function appear to be fully functional.

STM32: Add an IPv6 configuration for the STM32F4-Discovery board (witht he STM32-DISCO_BB base board).  Verify that the STM32 Ethernet driver works with IPv6.

Add IPv6 support to network driver skeleton and to SAMA5D4 Ethernet driver (which, unfortunately is still missing address filtering logi)

ICMPv6:  Add logic to behave like a router (if so configured):  NuttX will not send the router advertisement message in response to any router solicitation messages.

ICMPv6 Auto-configuration:  If no router reponds to the Router Solicitation, claim the link local address by sending the Neighbor Advertisement and return success

Networking:  Break out Ethernet definitions into a separate file; add IPv6 multicast addresses as common globals, Ethernet drivers need to filter link-local, all nodes Ethernet address