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>

package/base-files/files/etc/init.d/led

@@ -31,10 +31,11 @@ load_led() {
 	config_get gpio $1 gpio "0"
 	config_get inverted $1 inverted "0"
 
-	if [ "$trigger" = "rssi" ]; then
-		# handled by rssileds userspace process
-		return
-	fi
+	# execute application led trigger
+	[ -f "/usr/libexec/led-trigger/${trigger}" ] && {
+		. "/usr/libexec/led-trigger/${trigger}"
+		return 0
+	}
 
 	[ "$trigger" = "usbdev" ] && {
 		# Backward compatibility: translate to the new trigger