arm-bsp/image_types_disk_img: do not force 'ext4' FS by default

In some cases user doesn't want to create any file system on a specific partition.

Change-Id: I4596d97b95034952436cfd0a267a3f6ae4c0401d
Signed-off-by: Kamil Dziezyk <kamil.dziezyk@arm.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-bsp/classes/image_types_disk_img.bbclass

@@ -51,7 +51,7 @@ disk_img_createpart() {
     local imagefile="$1"
     local start="$2"
     local size="$3"
-    local fstype="${4:-ext4}"
+    local fstype="${4:-}"
     local content="${5:-}"
     local formatargs=""