Store the definition files in the initrd instead of reading them from
the Nix store in /sysroot.
This way, the initrd has to be re-generated every time the definition
files change. When the path to the defintion files instead of the
definition files themselves are embedded in the initrd, however, the
initrd also has to be re-generated every time. In this regard, this
change does not improve the status quo.
However, now systemd-repart also works reliable when the Nix store is
mounted separately from the root partition.
This change also enables new use-cases like creating partitions
necessary to boot the system. However, by default, the root partition
cannot be created on first boot because the systemd-repart service
requires a /sysroot to be mounted. Otherwise, systemd-repart cannot
determine the device to operate on.
Since v253, systemd-repart tries to create temporary directories in
/var/tmp. However, this directory doesn't exist in the initrd. This
commit adds an enviroment variable to re-use the existing /tmp directory
instead of /var/tmp.
