wandboard: Make explicit that a ext4 rootfs is used

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>