env: Rename environment to embedded_environment

The name 'environment' is widely used in U-Boot so is not a very useful
name of a variable. Rename it to better indicate its purpose.

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>