Ported Notification LED duration mod by @yank555-lu

Added entry in menuconfig to optionally add the mod to the kernel compile.

Use sysfs interface to change behaviour :

 * /sys/kernel/notification_leds/off_timer_multiplier (rw)
 *
 *   0 - Notification LED will blink forever until cleared by user
 *   1 - Notification LED will blink as requested by app or until cleared by user (default)
 *   2 to 255 - Notification LED will blink as requested by app multiplied by value or until cleared by user

NB: The output of the sysfs file (e.g. if you 'cat' it) will be clear text.