env: Make mmc as default option for CONFIG_ENV_FAT_INTERFACE

All configs are using mmc as default fat interface. That's why make it
default for everybody.
The reason for this patch is to make it default for Xilinx Zynq platform
which is not listed there.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
1 file changed