ARM: at91: sama5d3_xplained: save environment in a FAT file in MMC card

This patch will save U-Boot environment as a file: uboot.env, in FAT partition
instead of saving it in raw sector of MMC card.

This make us easier to manage the environment file.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Bo Shen <voice.shen@atmel.com>