Fix RAID detection for findiso

Chris Hofstaedtlerrequested to merge
zeha/live-boot:zeha/findiso-mdadm into master

mdadm 3.4-2 (Debian stretch) dropped the local-top mdadm script, and replaced it with a local-block script and some systemd parts.

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.

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

[zeha@d.o: forward upstream, reword]

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