mx25pdk: Align the environment with other FSL boards

Allow the boot of a device tree mainline kernel by aligning the environment
variables with other FSL boards.

Tested NFS boot of a dt 3.14-rc5 kernel.

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