wandboard: Make explicit that a ext4 rootfs is used

Make explicit that a ext4 rootfs is used.

As explained by Thomas Petazzoni:

"the ext4 filesystem is already used.
Indeed, the ext4 image is a symlink to the ext2 image, and the ext2
image is in fact an ext4 image:

-rw-r--r-- 1 thomas thomas 3763200 sept. 11 21:54 rootfs.ext2
lrwxrwxrwx 1 thomas thomas      11 sept. 11 21:54 rootfs.ext4 -> rootfs.ext2"

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/wandboard/genimage.cfg

@@ -2,7 +2,7 @@
 #
 # The SD card must have at least 1 MB free at the beginning.
 # U-Boot and its environment are dumped as is.
-# A single root filesystem partition is required (Ext2 in this case).
+# A single root filesystem partition is required (Ext4 in this case).
 #
 # For details about the layout, see:
 # http://wiki.wandboard.org/index.php/Boot-process
@@ -25,7 +25,7 @@ image sdcard.img {
 
   partition rootfs {
     partition-type = 0x83
-    image = "rootfs.ext2"
+    image = "rootfs.ext4"
     size = 512M
   }
 }