remove pools without a bootfs from BOOTFS variable
Use the same method used in zfs-load-key.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Llewelyn Trahaearn <WoefulDerelict@GMail.com>
Signed-off-by: Matthew Thode <mthode@mthode.org>
Closes #7089