env: Convert CONFIG_ENV_IS_IN... to a choice

At present we support multiple environment drivers but there is not way to
select between them at run time. Also settings related to the position and
size of the environment area are global (i.e. apply to all locations).

Until these limitations are removed we cannot really support more than one
environment location. Adjust the location to be a choice so that only one
can be selected. By default the environment is 'nowhere', meaning that the
environment exists only in memory and cannot be saved.

Also expand the help for the 'nowhere' option and move it to the top since
it is the default.

Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Move all of the imply logic to default X if Y so it works again]
Signed-off-by: Tom Rini <trini@konsulko.com>