Enable firstboot, disabled by default on Debian
Currently the first-boot conditions are not met by any Debian image (/etc/machine-id with content uninitialized, so we can just enable the build and ship it in the main package. This lets image builders (eg: cloud images) tinker with it.
https://www.freedesktop.org/software/systemd/man/machine-id.html#First%20Boot%20Semantics