portage-stable was updated to be in sync with upstream, so that the new
GLSA 201810-10, where systemd v238 got rejected due to security issues.
However, coreos-overlay still uses systemd v238. So we should also
follow the upstream strategy of whitelisting GLSA 201810-10, to build
Flatcar based on the current coreos-overlay.
So simply merge upstream/master into flatcar-master.