env: sunxi: Enable FAT-based environment support by default

Now that we have everything in place to implement the transition scheme,
let's enable it by default.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>