common: export overlay lower/upper/work directories as variables

This turns hard coded lower/upper/work dir names to configurable
variables.

Suggested-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

common/rc

@@ -244,7 +244,7 @@ _common_dev_mount_options()
 
 _overlay_basic_mount_options()
 {
-	echo "-o lowerdir=$1/lower,upperdir=$1/upper,workdir=$1/work"
+	echo "-o lowerdir=$1/$OVERLAY_LOWER_DIR,upperdir=$1/$OVERLAY_UPPER_DIR,workdir=$1/$OVERLAY_WORK_DIR"
 }
 
 _overlay_mount_options()
@@ -273,14 +273,14 @@ _overlay_mount()
 	local dir=$1
 	local mnt=$2
 	shift 2
-	mkdir -p $dir/upper
-	mkdir -p $dir/lower
-	mkdir -p $dir/work
+	mkdir -p $dir/$OVERLAY_UPPER_DIR
+	mkdir -p $dir/$OVERLAY_LOWER_DIR
+	mkdir -p $dir/$OVERLAY_WORK_DIR
 
 	$MOUNT_PROG -t overlay $SELINUX_MOUNT_OPTIONS \
-		    -o lowerdir=$dir/lower \
-		    -o upperdir=$dir/upper \
-		    -o workdir=$dir/work   \
+		    -o lowerdir=$dir/$OVERLAY_LOWER_DIR \
+		    -o upperdir=$dir/$OVERLAY_UPPER_DIR \
+		    -o workdir=$dir/$OVERLAY_WORK_DIR   \
 		    $OVERLAY_MOUNT_OPTIONS $* \
 		    $dir $mnt
 }