GRMLBASE/15-initsetup: provide workaround for failing rsyslog

Previous versions of the rsyslog package provided its rsyslog systemd
unit file as /lib/systemd/system/rsyslog.service. Starting with rsyslog
v8.2108.0-1, the file is provided as
/usr/lib/systemd/system/rsyslog.service though. This change is related
to the "dh_installsystemd: Prefer /usr/lib/systemd/ to /lib/systemd"
change from debhelper v13.4, which furthermore seems to be related to
the usrmerge situation.

Now executing `systemctl preset-all` fails though:

| # systemctl preset-all
| [...]
| Failed to preset unit, file /etc/systemd/system/syslog.service already exists and is a symlink to /lib/systemd/system/rsyslog.service.

By manually removing the /etc/systemd/system/syslog.service symlink, the
appropriate symlink (from /etc/systemd/system/syslog.service towards
/usr/lib/systemd/system/rsyslog.service) gets set up as expected again.

Reported towards Debian as #992847, but until it's fixed in the upstream
packaging, we need a workaround in our build tools.