zfsbootcfg: use vfs.root.mountfrom and update nextboot on every vdev

vfs.zfs.boot.primary_pool is only set on BIOS boot, use vfs.root.mountfrom
instead and update all vdevs on pool.

Reviewed by: allanjude
Sponsored by: Netflix, Klara Inc.
Differential Revision: https://reviews.freebsd.org/D25103