rules: make sure always set at least one property on rfkill devices

rules: make sure always set at least one property on rfkill devices

The rfkill service waits for rfkill device initialization as reported by
udev_device_is_initialized(), and if that is never reported it might dead-lock.

However, udev never reports completed initialization for devices that have no
properties or tags set. For some rfkill devices this might be the case, in
particular those which are connected to exotic busses, where path_id returns
nothing.

This patch simply sets the SYSTEM_RFKILL property on all rfkill devices, to
ensure that udev_device_is_initialized() always reports something useful and we
don't dead-lock.

Fixes: #2745