env: Support multiple environments

Now that we have everything in place to support multiple environment, let's
make sure the current code can use it.

The priority used between the various environment is the same one that was
used in the code previously.

At read / init times, the highest priority environment is going to be
detected, and we'll use the same one without lookup during writes. This
should implement the same behaviour than we currently have.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>