rssileds: use new application led trigger backend

rssileds: use new application led trigger backend

This will use the new application led trigger backend. For now this is
the only package that uses leds trigger in user space to configure the
kernel led triggers.

The callback script only emmits a message for now, so that this LED is now
managed by the rssileds service. Until now a generic warning was emitted that
this LED trigger is not supported. But that is not true.

-> Skipping trigger 'rssileds' for led '<name>' due to missing kernel module

I think this callback should be changed in the future to restart the
service.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>