Comment 10 for bug 1838525

When zeroing debconf value, I was able to see that grub-installer populates it again (grub-installer/bootdev with the wrong value "/dev/mapper"). The code that db_sets it again is:

    elif [ "$(device_to_disk "$cdsrc")" = "$default_bootdev" ] || \
       ([ -n "$hdsrc" ] && [ "$(device_to_disk "$hdsrc")" = "$default_bootdev" ]) || \
       ([ "$default_bootdev" = '(hd0)' ] && \
        (([ -n "$cdfs" ] && [ "$cdfs" != "iso9660" ]) || \
         [ "$hybrid" = true ])) || \
       ([ "$default_bootdev" != '(hd0)' ] && \
        ! partmap "$default_bootdev" >/dev/null && \
        ! grub_probe -t fs -d "$default_bootdev" >/dev/null); then
        db_fget grub-installer/bootdev seen
        if [ "$RET" != true ]; then
            bootfs=$(findfs /boot)
            [ "$bootfs" ] || bootfs="$(findfs /)"
            disk=$(device_to_disk "$bootfs")
            db_set grub-installer/bootdev "$disk" ---------- HERE
            state=2
        fi
    fi
    ;;