rfkill: Create rfkill-none LED trigger

Creates a new trigger rfkill-none, as a complement to rfkill-any, which
drives LEDs when any radio is enabled. The new trigger is meant to turn
a LED ON whenever all radios are OFF, and turn it OFF otherwise.

Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>