overlay: use OVL_BASE_SCRATCH_MNT instead of SCRATCH_DEV

overlay: use OVL_BASE_SCRATCH_MNT instead of SCRATCH_DEV

Use the new var OVL_BASE_SCRATCH_MNT to refer to overlay
base dir instead of the legacy SCRATCH_DEV var.

In overlay/011, when using MOUNT_PROG directly, provide
OVL_BASE_SCRATCH_MNT as device argument instead of 'none'.
This fix is required to support mount of base fs.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>