Add helper functions to access Nerves-set environment variables

[fhunleth]

There are cases where you want to build a project with elixir_make which handles all of the cross-compiler options itself. See https://elixirforum.com/t/building-nerves-project-with-a-dep-on-a-zephyr-project-built-with-elixir-make/65524/1 for a good example.

Rather than copy/paste all of the Nerves environment variables into every project, it would be nice to have a way to access this list. That way projects wouldn't have to adjust their lists each time Nerves is updated or risk an variable subtly changing a build.