mx53ard: Rework default environment to support FDT, MMC and netboot

This reworks the environment settings to be aligned with the other
i.MX boards. The loadaddr has been changed to allow the Freescale
kernel and mainline kernel to work without environment changes.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
mx53ard: 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>