Implement first part of the system update

* running the update will reboot the devvice into the system update mode (see
  http://freedesktop.org/wiki/Software/systemd/SystemUpdates/)
* in system update mode the run-update.sh script is executed through the systemd service
  which will later call the real update application but currently just creates a marker
  and reboots the device back into normal mode

Signed-off-by: Simon Busch <morphis@gravedo.de>