env: Create a new file for environment functions

At present we have environment.h but this file includes all the
environment-related header files as well as internals such as
default_environment.

It seems desirable to have a new header to hold the commonly used
environment functions, so that most files can avoid including all of this
unnecessary stuff.

Create a new env.h header and move one function over to it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>