Support XDG_DATA_HOME

[jjcarstens]

Resolves #120

[fhunleth]

I wouldn't expect us to dump Nerves hub files into the root of $XDG_DATA_HOME or am I misreading?

[jjcarstens]

Ooo. Right. I need to make a subdir

[jjcarstens]

Fixed. Its following the pattern of Nerves a bit. We can be more aggressive in following XDG standard here if we want as to not default .nerves-hub, or make that for a bigger release like we have done in other places. I'm indifferent.

[fhunleth]

I think that Nerves suffers from XDG_DATA_HOME apathy: we know it's the right thing to do, but no one cares enough to change it, and it is a breaking change to people who have XDG_DATA_HOME defined. I almost wonder if we've painted ourselves into a corner and if someone already has a ~/.nerves-hub directory that we should use it to avoid confusing them. Or give them a warning and tell them to type mv ~/.nerves-hub $XDG_DATA_HOME/nerves-hub. Ugh.

[jjcarstens]

Okay, I think I like how this is working. Application and env var get precedence. If it comes to the default and ~/.nerves-hub exists but the default doesn't, then it will fail with an error to give a chance for quick migration.

The only other thing to consider is if we want to travers XDG_DATA_DIRS looking for our possible config and attempt to use that. I feel like this might be a bit overkill, but...maybe thats more complete for the standard?

[fhunleth]

Suggested change

	xdg_home_nh = :filename.basedir(:user_data, "nerves-hub", %{os: :linux})
	xdg_home_nh = :filename.basedir(:user_data, "nerves-hub")

It seems like we should trust whatever basedir tells us that we should use for storing data so that this does the right thing on OSX. I "think" that renaming the variables to remove the XDG and updating the error text to refer to this as the user data directory below is what's needed to make this generic.

I.e., the same logic can still be used:

1. Use Application.get_env(:nerves_hub_cli, :home_dir)
2. Use $NERVES_HUB_HOME
3. If ~/.nerves-hub and not the :user_data location, tell the user to move the directory
4. Use the :user_data location