This stage runs a given command only on the first boot of the image,
useful for doing instantiation tasks that can only be done in the
target environment, or that should be done per-instance, rather
than per image.
Ideally we would use systemd's ConditionFirstBoot for this, but that
requires images to ship without an /etc/machine-id, and currently
we only support shipping images with an empty /etc/machine-id.
Changing this would mean dropping /etc/fstab in favor of mounting
the rootfs rw from the initrd. This is likely the right thing to
do regardless, but we would have to audit what other first-boot
services we would end up with pulling in in this case.
Instead we introduce our own flag file /etc/osbuild-first-boot,
and use ConditionPathExists.
Signed-off-by: Tom Gundersen <teg@jklm.no>
c2243aee6a