Automatically move configuration files to persistent storage

This patch adds ETC_FILES_AUTO that contains a list of configuration
files that are moved to persistent storage during disk initialization.

It also adds a systemd service file to scan the persistent storage for
configuration files during boot. Minor changes were necessary so that
psdk exits cleanly when no persistent disks are found and when no
changes are required.

On first boot after configuring a persistent disk, systemd will show:

 user@apertis:~$ sudo systemctl status psdk-conf-scan.service
 ● psdk-conf-scan.service - Scan for configuration files on persistent storage
    Loaded: loaded (/etc/systemd/system/psdk-conf-scan.service; enabled; vendor preset: enabled)
    Active: active (exited) since Mon 2019-06-17 09:06:39 UTC; 19s ago
   Process: 473 ExecStart=/usr/bin/psdk -s (code=exited, status=0/SUCCESS)
  Main PID: 473 (code=exited, status=0/SUCCESS)

 Jun 17 09:06:39 apertis sudo[538]: pam_unix(sudo:session): session opened for user root by (uid=0)
 Jun 17 09:06:39 apertis sudo[538]: pam_unix(sudo:session): session closed for user root
 Jun 17 09:06:39 apertis sudo[547]:     root : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/bin/mv -f /etc/cntlm.conf /etc/cntlm
 Jun 17 09:06:39 apertis sudo[547]: pam_unix(sudo:session): session opened for user root by (uid=0)
 Jun 17 09:06:39 apertis sudo[547]: pam_unix(sudo:session): session closed for user root
 Jun 17 09:06:39 apertis sudo[551]:     root : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/bin/ln -sf /home/etc/cntlm.conf /etc
 Jun 17 09:06:39 apertis sudo[551]: pam_unix(sudo:session): session opened for user root by (uid=0)
 Jun 17 09:06:39 apertis sudo[551]: pam_unix(sudo:session): session closed for user root
 Jun 17 09:06:39 apertis psdk[473]: Using configuration file found on persistent storage. From now on, changes made to /etc/cn
 Jun 17 09:06:39 apertis systemd[1]: Started Scan for configuration files on persistent storage.

And then on following boots:

 user@apertis:~$ sudo systemctl status psdk-conf-scan.service
 ● psdk-conf-scan.service - Scan for configuration files on persistent storage
    Loaded: loaded (/etc/systemd/system/psdk-conf-scan.service; enabled; vendor preset: enabled)
    Active: active (exited) since Mon 2019-06-17 09:07:59 UTC; 5s ago
   Process: 471 ExecStart=/usr/bin/psdk -s (code=exited, status=0/SUCCESS)
  Main PID: 471 (code=exited, status=0/SUCCESS)

 Jun 17 09:07:59 apertis systemd[1]: Starting Scan for configuration files on persistent storage...
 Jun 17 09:07:59 apertis sudo[545]:     root : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/usr/bin/find /home/etc -type f
 Jun 17 09:07:59 apertis sudo[545]: pam_unix(sudo:session): session opened for user root by (uid=0)
 Jun 17 09:07:59 apertis sudo[545]: pam_unix(sudo:session): session closed for user root
 Jun 17 09:07:59 apertis psdk[471]: /etc/cntlm.conf is already on persistent storage. No action needed.
 Jun 17 09:07:59 apertis systemd[1]: Started Scan for configuration files on persistent storage.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>