config_distro_bootcmd: Run 'scsi scan' before trying scsi disks

Scsi disks need to be probed before we try to access them, otherwise all
accesses fail with: ** Bad device size - scsi 0 **.

Reported-by: Karsten Merker <merker@debian.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Karsten Merker <merker@debian.org>