Comment 3 for bug 1302772

You should be able to work around this by editing /etc/maas/pserv.yaml and commenting out, or removing, the entire section called “boot” before the upgrade.

We should have kept the “boot” section of the schema (at least as it appeared in Saucy, before it grew fancy) just for compatibility. I think I first added those back for compatibility when somebody else had removed them, and then later stupidly removed them again myself.