Cope with / being on a ZFS root dataset

Cope with / being on a ZFS root dataset

If / is on the root dataset in a ZFS pool, then ${bootfs} will be set to
"/" (whereas if it is on a non-root dataset, there will be no trailing
slash).  Passing "root=ZFS=${rpool}/" will fail to boot, but
"root=ZFS=${rpool}" works fine, so strip the trailing slash.

Fixes: https://savannah.gnu.org/bugs/?52746
Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Tested-by: Fejes József <jozsef.fejes@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>