Provide an "offline" way of updating WiFi credentials

[tmigone]

Currently the device can get into an unrecoverable state in the following scenario:

• The device has no internet access via ethernet port
• You provided no WiFi network credentials, those credentials are wrong, or the target WiFi network is not available

The device in this case will not be reachable so the only means to recovering it is plugging in ethernet cable, or wait for the WiFi network to reestablish if that was the problem.

Some solutions for this problem could be:

• use WiFi Connect to get credentials and connect (a quick test yielded conflicts in DBUS handling because both WiFi Connect and WiFi Repeater are talking to Network Manager at the same time). WiFi Connect also needs to be patched to allow scanning with one interface and using another to connect to WiFi.
• provide a mechanism to prevent the WiFi Repeater's AP to start. For example, if GPIO 2 is 0v then don't create the AP. This will solve the problem for cases where the initial provisioning WiFi network is available
• implement functionality similar to what WiFi Connect offers where you grab the WiFi networks available and present a basic UI to fill in credentials
• implement a webportal similar to what finternet project does.