systemd instantiated services are not started when enabled via ignition

[coreos]

systemd instantiated services are not started when enabled via ignition

[dustymabe]

picking up the conversation from systemd/systemd#9901 (comment)

@ajeddeloh
I'm not sure about how to approach adding support for this in Ignition itself. Probably via a compile time switch like "systemdversion=240" which lets us enable things only supported in 240+. We can't just enable it because then it will write out invalid files for anything that doesn't have systemd new enough.

Since at the point we are writing systemd unit files we have the real root mounted, can we check the version of systemd in the real root and act appropriately (i.e. at runtime).

[peterbaouoft]

PR in systemd/systemd#9901