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>

board/wandboard/genimage.cfg

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