This causes problems if, say, you install a kernel after running fedup (but before starting the upgrade).
Instead we should use a systemd generator to temporarily change the default unit if certain flag files exist - like systemd's SystemUpdates feature does.
Probably the flag file(s) should be written by the fedup initrd so the upgrade won't start unless you've booted our special image.
This causes problems if, say, you install a kernel after running fedup (but before starting the upgrade).
Instead we should use a systemd generator to temporarily change the default unit if certain flag files exist - like systemd's SystemUpdates feature does.
Probably the flag file(s) should be written by the fedup initrd so the upgrade won't start unless you've booted our special image.