sunxi: Transition from the MMC to a FAT-based environment

sunxi: Transition from the MMC to a FAT-based environment

The current environment has been hardcoded to an offset that starts to be
an issue given the current size of our main U-Boot binary.

By implementing a custom environment location routine, we can always favor
the FAT-based environment, and fallback to the MMC if we don't find
something in the FAT partition. We also implement the same order when
saving the environment, so that hopefully we can slowly migrate the users
over to FAT-based environment and away from the raw MMC one.

Eventually, and hopefully before we reach that limit again, we will have
most of our users using that setup, and we'll be able to retire the raw
environment, and gain more room for the U-Boot binary.

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