env: Allow to build multiple environments in Kconfig

Now that we have everything in place in the code, let's allow to build
multiple environments backend through Kconfig.

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