env: Rename environment.h to env_internal.h

This file contains lots of internal details about the environment. Most
code can include env.h instead, calling the functions there as needed.

Rename this file and add a comment at the top to indicate its internal
nature.

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>
[trini: Fixup apalis-tk1.c]
Signed-off-by: Tom Rini <trini@konsulko.com>