Add fwup task to reprovision devices
Add fwup task to reprovision devices

The provision task lets you use `fwup` to re-apply the provisioning
information to a MicroSD card. For example, if you store initial WiFi
credentials using the provisioning feature, then to change them, put the
MicroSD card back in your computer and run:

```sh
NERVES_WIFI_SSID="new_ssid" NERVES_WIFI_PASSPHRASE="new_password" fwup
firmware.fw -t provision
```

This only writes the provisioning data. Nothing else on the MicroSD card
will be changed.

nerves-project/nerves_system_rpi3,nerves-project/nerves_system_rpi3