board/udoo/neo: specify an offset for the rootfs partition

When manually writing SPL and u-boot.img to the SD card the rootfs
gets corrupted.

Fix this issue by passing a 1MB offset for the rootfs.

Reported-by: Breno Lima <breno.lima@nxp.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Breno Lima <breno.lima@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

board/udoo/neo/genimage.cfg

@@ -26,6 +26,7 @@ image sdcard.img {
   partition rootfs {
     partition-type = 0x83
     image = "rootfs.ext4"
+    offset = 1M
     size = 512M
   }
 }