wifi: show devices behind a firewall that are connected to the  WiFi hotspot

* Clients that are behind a firewall, in particular Windows-PCs tend
to not respond to the request invoked by the isReached(val)-method.
Executing a ping instead and evaluating the outcome enables us to
bypass the firewall.

Change-Id: If857545c7a4d5fbc9d2dc919096a9c84e64d6877