default link up on loopback interface

Since the loopback interface doesn't have a cable, I think it should always indicate "link up" instead of staying down all the time.
LwIP has no use for this internally yet, but there are check functions for link and I think loopback should always return TRUE on those test functions.
(Also I use it in a modifed version of the ip_route() function to not even include netifs without link in the routing)

closes #12