env: Drop the ACTION typedef

Avoid using a typedef here which is unnecessary. Add an 'env_' prefix to
both the enum and its members to make it clear that these are related to
the environment.

Add an ENV prefix to these two flags so that it is clear what they relate
to. Also move them to env.h since they are part of the public API. Use an
enum rather than a #define to tie them together.

Signed-off-by: Simon Glass <sjg@chromium.org>