Refactoring build configurations

[Frank Hunleth]

All,

I've been refactoring the build configurations to remove replicated
code and to enable Elixir by default. Elixir doesn't add anything to
the target footprint if you don't use it so it's harmless to enable in
the SDK. This change lets me not have to keep duplicate
Elixir/non-Elixir configurations. The other changes are minor things
like moving the 3 identical busybox configurations to the
nerves-common directory and aligning file naming. When you update, you
may want to double check your configuration if you've been modifying
it locally.

Also, I have a Raspberry Pi build for Nerves in a branch that I'll be
moving to master as soon as I get a bug fixed in Buildroot. After
that, I'll probably be done with adding more platforms for a while and
will be focusing on cleaning up and adding functionality to the
current platforms.

Frank

--
Frank Hunleth
Troodon Software LLC
Embedded Software Development
http://troodon-software.com/