udev: allow different attribute values for the community firmware

When using the community-maintained firmware, some attributes need to be set with different values than what works best with the stock firmware: this is the case of the power/control attribute which should be set to "on". Similarly, on the PPP power/persist should be set to "1".

This commit reworks the udev rules so we can differentiate between firmwares and host devices.

Supersedes !49 (closed)

Fixes #36 (closed)

Untested for now, hence the "Draft"!