turn autoconf back on for v6_mapped ifaces

The SLAAC-removal commit earlier this week was
naive and won't work.  Restore previous behavior
until a better commit, as otherwise there might
be strange fallout when the current SLAAC addrs
eventually expire.

Change-Id: If08f1d29c6a54a1bce6aac8e2f5b020b74f1849e