Fix boot failure in Dracut scripts.

Fix boot failure in Dracut scripts.

mount-zfs.sh script incorrectly parsed results from zpool list.  Correct
bootfs attribute was only found on systems with a single pool or where
the bootable pool's name alphabetized to before all other pool names.
Boot failed when the bootable pool's name came after other pools
(IE 'rpool' and 'mypool' would fail to find bootfs on rpool.)

Patch correctly discards pools whose bootfs attribute is blank ('-').

Signed-off-by: Zachary Bedell <zac@thebedells.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #402