base-files: add new application led trigger backend

base-files: add new application led trigger backend

For now we have only kernel LED trigger support. With this change it is now
possible to use application triggers.

If we configure a LED with a non kernel trigger, then we check on every
restart and boot of the LED service if we have this trigger as an application
in "/usr/libexec/led-trigger". If this file with the name is found, then we
execute this to init the LED.

Possible use cases are:

- Start/Stop/Restart an application led trigger service for this led
- Init a LED that is configured by a hotplug script (VPN tunnel established)

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