UBI: add layout volume information

Add more information about layout volume to make userspace tools
use the macros instead of constants. Also rename UBI_LAYOUT_VOL_ID
to make it consistent with other macros.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/drivers/mtd/ubi/scan.c b/drivers/mtd/ubi/scan.c
index 6f2680f..05aa3e7 100644
--- a/drivers/mtd/ubi/scan.c
+++ b/drivers/mtd/ubi/scan.c
@@ -858,7 +858,7 @@
 	}
 
 	vol_id = be32_to_cpu(vidh->vol_id);
-	if (vol_id > UBI_MAX_VOLUMES && vol_id != UBI_LAYOUT_VOL_ID) {
+	if (vol_id > UBI_MAX_VOLUMES && vol_id != UBI_LAYOUT_VOLUME_ID) {
 		int lnum = be32_to_cpu(vidh->lnum);
 
 		/* Unsupported internal volume */