Cut erlang.mk into many small components

 *  The build.config says what gets into the generated erlang.mk.
 *  The default erlang.mk in the repository hasn't changed yet.
 *  Clean targets were separated into "clean" and "distclean".
 *  The "help" target was added to display some help message.

I probably broke a couple things...
Change GitHub raw file URL

GitHub issues a 301 Moved Permanently response on all calls to
`https://raw.github.com`, which redirects to
`https://raw.githubusercontent.com`.