The next-node MAC address doesn't need to be unique in different
communities, so we can as well add a default value.

The SSID for the mesh network should be cryptic, otherwise some users will try to connect to that network and have a bad user experience.

The used nodejs 4.4.5 needs at least GCC 4.8 to be build
https://github.com/nodejs/node/blob/v4.x/BUILDING.md



Signed-off-by: Matthias Fritzsche <matthias@chemnitz.freifunk.net>
[Matthias Schiffer: moved information to the first section]

iptables will be pulled in anyways.

The Airgateway LR is the same device as the normal Airgateway but with an detachable antenna. The Pro version is a cheap dual band device.

gluon-core: dns-cache documentation polish
Now its clear that upstreams severs normally means "community gateways".
* added cacheentries to description
* made upstream server description more general
* description of the A and AAAA-records for next_node added
* mention of Cache not being allocated when not set in site.conf 
* dns-cache doku polish

For simplicity, we don't use different MTUs for compat 14 and 15 anymore,
there's no harm in using 1532 for batman-adv-legacy as well.

The fastd_mesh_vpn site.conf section is renamed to mesh_vpn.fastd.