Don't rely on the first boot to finish installation

[bamarni]

On early versions of pi64 (based on Jessie), all packages were getting installed on a chroot containing qemu.

However since the upgrade to Stretch this started to fail as they made systemd not included by default and installing this package was failing with qemu. So I ended up postponing this step to run on the pi itself, on the first boot (essentially it's dpkg --configure -a).

This is not optimal as it makes the first boot last a few minutes, and might corrupt the system if the user makes a hard shutdown during the process.

Ideally that logic would be moved upfront, if it ends up being not possible, the release process should have a second step involving a raspberry pi or a cloud server such as https://www.scaleway.com/. However that'd make it more complicated for users to build pi64 from source...