Add support for external LED application.
Add support for a callback script/application to control LEDs.
This commits adds the ability to control a LED via an external application. The API is simple and is described in 'iopsys-brcm63xx-arm/tiger/base-files/sbin/external_led_ctrl.sh'.
To avoid unnecessary execution of the callback-script, logic has been added to keep track on when to update the LEDs.