Can't upgrade jails to 8.0 using freebsd-update

[remodeler]

I don't think your /usr/jail/mysql/ root is really a basedir, the way
freebsd-update thinks of it. The jail only has world files, not the kernel.

There are a number of specialized jail admin tools in ports. Also, the
handbook has good information on a manual method to maintain jails, using
symlinks and nullfs mounts for security. It uses make installworld with a
target to the jail directory.