env: Allow env_attr_walk to pass a priv * to callback

env: Allow env_attr_walk to pass a priv * to callback

In some cases it can be helpful to have context in the callback about
the calling situation. This is needed for following patches.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>