Update: this was a suspected race condition when initially encountered, however (1) the behavior was not reproduced by emulating the conjectural race (2) this was during a brief window between merging the revised image tools and merging some remaining supporting tools, so may have been a transient issue.

The vyos-grub-update.service unit is set After=local-fs.target, yet this would appear not to guarantee the existence of the persistence mount, required by grub_update.py for normal functioning. Add mechanism to ensure this mount exists before unit is run.