0020-read-only: iterate over all devices + improve console message layout

There might be block devices other than /dev/sd* + /dev/vd* (like /dev/nvme0n*). Instead of hardcoding a static list, let's iterate over all of them, and then ignore all symlinks (pointing to the actual devices) and consider only actual block devices. Also ensure that the device exists (checked via blockdev --getsz ...) before executing blockdev --setro ... on it.

Reworked the according console message: using printf with field width allows us to make this look a bit nicer and better integrate into the boot sequence look-alike.

JFTR: it would be nice to send output also to /boot.log (which ends up as /var/log/live/boot.log on the booted system), but if we do this then the output is visible twice during startup, as /boot.live is sent to console via tail -f. If changing this to only list it via /boot.live it might not be always guaranteed to be visible on screen, so we didn't change this yet.

This work was funded by Grml-Forensic.

[zeha@d.o: forward upstream]