common: Move environment choice to Kconfig

common: Move environment choice to Kconfig

The environment location is something that might change per board
(depending on what storage options are availaible there) or depending on
the user choice (when we have several options).

Instead of hardcoding it in our configuration header, create a Kconfig
choice with the options we use for now, and the symbols that depend on it.

Once done, also remove the irrelevant sunxi defines.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>