[feature] Add device recognition mechanism

[nemesifier]

Due to possible misconfiguration or bugs, in theory it's possible that the firmware upgrader could reflash the wrong device.

Since this is theoretically possible, we should add a mechanism which would be able to spot this case and prevent it from happening.

I immagine it as a method, which would have to be adapted for each upgrader backend, on OpenWRT we could simply check the uuid of the device in /etc/config/openwisp matches the UUID of the device currently being upgraded.

If the method spots this, it should raise an UpgradeAborted error but we must ensure the reason is clearly communicated to the user.