If devicepersistency="by-label" is set in the KIWI description
it will correctly operate on the fstab values but still uses
the UUID based setting for root= in the grub config. This commit
allows to set root=LABEL=... in the grub config in case the
devicepersistency requested it. In order for this to work this
commit also had to increase the scope of the grub helper
method _fix_grub_root_device_reference which is now called in
any case. This Fixes#1757