Allow configuration of device string

As inaris have variable device strings this allows configuration of a device identifier so we can pass the correct device to the cert install scripts.

```ini
[flash]
flash_device = inari
flash_device_id = <whatever is reported by adb devices -l>
```

For inaris you want the string prefixed with usb: 

e.g.

`--flash_device_id 'usb:whatever'`