Don't generate rootfs entry in /etc/fstab if we don't know its UUID [Closes: issue2182]

With a command line like:

  grml-debootstrap --grub /dev/sda -t /media/sda1 ...

the resulting fstab gets:

  /media/sda1 /  auto    defaults,errors=remount-ro 0   1

which is for sure unwanted. So don't such an entry in the fstab
if we lack details about the underlying rootfs and instead warn
the user that this *might* be a problem.