A udev-based device enumerator is not sufficient, since libudev is an
optional dependency, or udev might fail. In these cases, we should fall
back to using sysfs to enumerate devices.
Add a DeviceEnumeratorSysfs class which is a specialization of
DeviceEnumerator that uses sysfs to enumerate media devices on the
system.
Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
