zfsboot reads one time boot directives from ZFS pool Pad2 area.
The area was previously described as "Boot Block Header", but currently
it is know as Pad2, marked as reserved and is zeroed out on pool creation.
The new code interprets data in this area, if any, using the same format
as boot.config. The area is immediately wiped out.
Failure to parse the directives results in a reboot right after
the wipe out. Otherwise the boot sequence proceeds as usual.
zfsbootcfg writes zfsboot arguments specified on the command line to the Pad2
area of a disk identified by vfs.zfs.boot.primary_pool and
vfs.zfs.boot.primary_vdev kenv variables that are set by loader during boot.