nerves: follow XDG base dir specification

[fhunleth]

Nerves stores global data to ~/.nerves. The XDG Base Dir spec provides a standard location for these kinds of files. There's an Elixir issue and discussion that have some compelling reasons behind changing. It seems like Nerves should change as well. See:

• mix: follow XDG base dir specification elixir-lang/elixir#8818
• https://groups.google.com/forum/#!topic/elixir-lang-core/7nyDfkgRsI0

[mobileoverlord]

Starting in Nerves 1.5.2
the data directory for Nerves will use XDG_DATA_HOME if set. We will revisit supporting the XDG spec more closely in a major version upgrade.