allow to toggle flashlight via sysfs

adds a config file, which can be used in device trees as overlay to
specify the sysfs path to the flashlight.

this allows to toggle flash on and off as well and is much faster than
using the camera.

Change-Id: Ibd1a8becf5e09e421c44e427bed00d01ed092162
Signed-off-by: Alexander Martinz <eviscerationls@gmail.com>