Removed suggestion to use root dataset as bootfs

The dracut howto proposed to boot from the root dataset of a pool.
Apart from this giving problems when booting (as the code seems to
expect a child dataset and creates an illegal dataset name when using
the root dataset) the technical limitations of the root dataset
(among others the inability to rename or destroy through the zfs
command) resulted in the general consensus to only use it as a
container for the datasets in the pool - not as a filesystem itself.

Removed the idea to boot from the root dataset.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: bunder2015 <omfgbunder@gmail.com>
Signed-off-by: Gregor Kopka <gregor@kopka.net>
Closes #8247