overlay: fix _overlay_config_override of MOUNT_OPTIONS

overlay: fix _overlay_config_override of MOUNT_OPTIONS

The config variable OVERLAY_MOUNT_OPTIONS is used to configure
the overlay mount options when running ./check -overlay.
The config variable MOUNT_OPTIONS is used to configure the
mount options for base fs.

If config sets value of OVERLAY_MOUNT_OPTIONS and
does not set MOUNT_OPTIONS, the value of MOUNT_OPTIONS
may be leftover from previous _overlay_config_override, so
don't use that value for base fs mount.

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