Avoid deadlock during boot by using systemctl --no-block.

By default systemctl waits until the requested action has been completed,
however live-config itselfs runs as a service managed by systemd and
the requested action cannot complete until live-config itself has
finished... this creates a deadlock during boot.

To avoid this we use the --no-block option of systemctl which
instructs systemd to enqueue the requested operation and systemctl
to not wait its completion.