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.
Output the amended repart definitions to a well-known directory in
$TMPDIR instead of using a temporary directory with a random directory
name.
The output file `repart-output.json` also contains the full path to the
repart definition file used to create the partition. As
`amend-repart-definitions.py` uses `tempfile.mkdtemp`, this introduces
an impurity:
```json
{
"type" : "root-x86-64",
"label" : "rootfs",
"uuid" : "f2fa2e49-e443-45d2-a2e2-c3754cab6363",
"file" : "/build/tmppjo7kv5o/rootfs.conf",
"node" : "image.raw2",
"offset" : 135266304,
"old_size" : 0,
"raw_size" : 1651101696,
"old_padding" : 0,
"raw_padding" : 0,
"activity" : "create",
}
```
This commit changes the parent directory of the amended repart
definitions to `/build/amended-repart.d/`.
Write the output of `systemd-repart` as a JSON file to
`$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).
