Add option to enable Aduino Due specific logic, like pull-up on UART0 RXD.  Suggested by gdi@embedders.org

Make net_close() nonblocking and free unestablished connections if no free connections available.  From Max Holtzberg

Another case where we should use the routing table: There may be multiple routers on the same network.  In that case, we have to search for the routing table for the router that forwards to the destination address.  We do this when we determine the MAC address we want in the ARP logic

Fix ordering of operations in network ioctl handling.  We need to able to distinguish an error because the command was not recognized vs. other kinds of error

netdev_findbyaddr() will not use a routing table if one is available.  It will also make a better decision in the case there there is no routing table but only a single network interface.

Add framework for Spark Core board support.  The initial commit is a clone of the Maple Mini and still needs Spark customizations