Allow giving a custom package containing the `systemd-repart` binary.
Defaults to `pkgs.systemd`. This option opens up the possibility to use
a different package for the image builder and the system configuration.
For example, someone could use this option to build an image with a
patched systemd while still using the upstream nixpkgs systemd package
(i.e., `pkgs.systemd`) for the system configuration installed to the
created image.
Write the output of `systemd-repart` as a JSON file to
`$out/repart-output.json`.
Depending on the repart configuration, the output of `systemd-repart`
contains important information, for example, when creating verity
partitions:
> The verity root hash itself will be included in the output of
> systemd-repart.
See `Verity=` in repart.d(5).
