Resolve node id from gateway mac address.
Resolve node id from gateway mac address.

As discussed in plumpudding/hopglass#42.

simonkurka/hopglass-server,simonkurka/hopglass-server,freifunk-saar/hopglass-server,hopglass/hopglass-server,plumpudding/hopglass-server,eberhab/hopglass-server