Expired root user

[Gylstorff Quirin <quirin.gylstorff@siemens.com>]

Hi all,

in meta-iot2050[1] the password of the root user is set to expired to 
force the user to reset the password during the first boot. This 
currently done in a postinst [2] by calling:

```
passwd --expire root
```

An alternative way would be the setting:

```
USER_root[expire] = "1970-01-01"
```

Both variants have the issue that the installation of packages which add
new users will fail with a error message similar to:

```
Setting up systemd (247.3-7) ...
Created symlink 
/etc/systemd/system/getty.target.wants/getty@tty1.service -> 
/lib/systemd/system/getty@.service.
Created symlink 
/etc/systemd/system/multi-user.target.wants/remote-fs.target -> 
/lib/systemd/system/remote-fs.target.
Created symlink 
/etc/systemd/system/sysinit.target.wants/systemd-pstore.service -> 
/lib/systemd/system/systemd-pstore.service.
Initializing machine ID from random generator.
Your account has expired; please contact your system administrator.
chfn: PAM: Authentication failure
adduser: `/bin/chfn -f systemd Network Management systemd-network' 
returned error code 1. Exiting.
dpkg: error processing package systemd (--configure):
installed systemd package post-installation script subprocess returned 
error exit status 1
Setting up dmsetup (2:1.02.175-2.1) ...
Errors were encountered while processing:
systemd
E: Sub-process /usr/bin/dpkg returned an error code (1)
WARNING: exit code 100 from a shell command.
```

Possible solutions are:
- moving the account creation / modification to the rootfs postprocessing
- using the systemd first boot service[3] for changing the root password

Claudius was there are reason why the accounts are created/modified 
before installing the rootfs? You add the functionality with
163f50 meta/classes: add image-account-extension class

Quirin

[1]: https://github.com/siemens/meta-iot2050
[2]: 
https://github.com/siemens/meta-iot2050/blob/master/recipes-core/customizations-example/files/postinst
[3]: 
https://www.freedesktop.org/software/systemd/man/systemd-firstboot.html#