Comment # 11 on bug 1198668 from Michal Koutn���

Quick check would be
> systemctl show -p Before purge-kernels.service
(possibly traverse further until you reach systemd-user-sessions.service or
default.target (graphical.target))

Alternatively, you may find the ordering path(s) in a plot
> systemd-analyze dot --order | dot -Tsvg >systemd.svg
(warning: kind of firehose drinking)

Thirdly, I can see the purge-kernels.service is of Type=oneshot, which means
it'll block subsequent jobs until the ExecStart= command finishes. Not sure if
there's actually necessity to wait after kernel removal is done, the
Type=simple or Type=exec would only wait for the command start and the removal
may run in parallel (even with jobs that specify After=purge-kernels.service).

Additionally, there may be extra-systemd implicit dependency (such as the
zypper lock) that prevents boot progression.