wandboard: Change default environment to cope with OE changes

OpenEmbedded has change partitioning layout of generated image so it
does not raise warnings during the boot regarding unkown partition
being used for U-Boot.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Fabio Estevam <fabio.estevam@freescale.com>