9990-misc-helpers.sh: fix RAID detection for findiso.

The local-top mdadm script was replaced with:
  1.) systemd integration
  2.) a local-block mdadm script for initramfs-tools-based setups
in Debian stretch, so this part was broken ever since.

Use the "new" local-block mdadm script instead.

This one won't allow us to select a specific device to assemble, so just
let it start all of them and hope for the best.

Closes: https://github.com/grml/grml/issues/125