Add check for bridge

[arve0]

This will allow restarting bridge connection. Hack for #2028.

if (Bridge.isOk() != true) {
  Serial.println("waiting..");
  delay(60000); // hope for ar9331 booting
}
// restart bridge
Bridge.started = false;
Bridge.begin();

[facchinm]

As in #2781, I did not merge this PR because it's more like an hack than a real fix.
In fact, the "XX" command you are sending triggers this function (https://github.com/arduino/YunBridge/blob/master/bridge/packet.py#L98) which launches usr/bin/blink-start (unwanted) and returns 2.
A real "ping" function would be a good improvement but must be implemented by both side of the Bridge; on the python side it will be something like

self.processor.register('Q', PING_Command(self))

I compiled a list of commands already in use https://gist.github.com/facchinm/569c1c8d112d28d8ff73 so if you want to update your merge request dividing it in two parts (one here and one for YunBridge) while keeping the same functionality it would be great!

[arve0]

Thanks for the feedback. I might look into creating a correct ping, if I find some time.