[packages] Add unreachable rules for tunl0

Always set tunl0 as network interface for freifunk firewall zone (fixes #133)
This ensures that freifunk-policyrouting adds unreachable rules for tunl0
otherwise traffic may leak if a SmartGateway client creates a tunnel.
After firstboot ip ruleset looks now like the following:

$ ip rule
  0:      from all lookup local
  1000:   from all lookup olsr
  2000:   from all lookup localnets
  19999:  from all iif tunl0 lookup olsr-tunnel
  20000:  from all iif tunl0 lookup olsr-default
  20001:  from all iif tunl0 unreachable
  32766:  from all lookup main
  32767:  from all lookup default