In the past user had to customize Kubernetes or use a bind mount to make
writing the default /usr/libexec/kubernetes/ path work. With
systemd-sysext on by default the bind mount doesn't work anymore because
it can get lost. A newer workaround is to use a systemd-sysext image
that creates a symlink in /usr/libexec/... to redirect to somewhere
under /var/.
Instead of relying on workarounds, make Kubernetes work by default on
Flatcar by having the symlink be part of the generic image. The target
folder will be created through a tmpfiles rule.
