sixlowpan: ignore incoming packets if unconfigured

If there's no IPv6 address configured so far to any interface, 6lowpan
should not try to  handle incoming packets. This can easily lead to
looping packets.
sixlowpan: documented internal is_our_address()
Merge pull request #1606 from OlegHahm/sixlowpan_ignore_unconfigured

sixlowpan: ignore incoming packets if unconfigured