wandboard: genimage: Pass an offset for the rootfs

Pass an offset of 1MB for the start of the rootfs.

Otherwise we get rootfs corruption when the bootloader is manually
written to the SD card.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 82c1445fc473cf422a1283283acfa46f0df6cd84)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/wandboard/genimage.cfg

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