Change the default IPv6 prefix from aaaa::0/? to fc00::0/7

Ever since 2008, the Contiki code has used an unauthorized IPv6 prefix `aaaa::0/?`, which at the time was picked for lack of a better prefix.

This pull request changes the default prefix to `fc00::0/7`, which is the unique local prefix as defined by RFC4193. Unique local addresses should not be globally routed, but may be used internally within a network or an organization, and makes a lot of sense to use as the default prefix in Contiki.