/var/lib/system-image must be present in packaging

[Barry Warsaw]

The packaging branch for the client needs to create /var/lib/system-image because the client cannot do this when it runs. The file system is read-only at that point, but if the directory is added when the package is installed, an empty persistent directory will be bind-mounted on top of it.

[Stéphane Graber (stgraber)]

So it looks like the new packages indeed contain /var/lib/system-image, including /var/lib/system-image/keyrings.

However on my device, I mount an empty directory on /var/lib/system-image on first boot, which means that the keyrings directory can't be found by system-image-cli and results in:
FileNotFoundError: [Errno 2] No such file or directory: '/var/lib/system-image/keyrings/image-master.tar.xz'

system-image-cli needs to be changed to create the keyrings directory when missing.