userspace/rootfs_override: save date of last update

Note: Be aware that date of last update may be not accurate.
One reason is that it uses date stored before last restart, so time needed to restart is not taken into account.
Additionally date stored before restart may be not accurate for many reasons like:
--hard restart (last know date was not saved)
--date was never received from ntp nor set manually
Signed-off-by: Adam Wujek <adam.wujek@cern.ch>