ARM: at91: at91sam9n12ek: save the environment to a fat file in MMC card

Insteading in mmc's raw sectors, this patch will save the environment
in a fat file (uboot.env) in mmc card's first FAT patition by default.

If you want to save in mmc's raw sectors, you only need to define
CONFIG_ENV_IS_IN_MMC.

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