Use XDG_CONFIG_HOME for .platformio directory

Temporary solution

You can override default location of .platformio folder using core_dir configuration option.

It is also possible to override the .platformio directory with the PLATFORMIO_CORE_DIR environment variable.

Configuration

Operating system: macOS 10.14

PlatformIO Version (platformio --version): 4.0.1b3

Description of problem

The ~/.platformio directory contains important config data including the user's custom boards. It's currently one of many hidden directories in my home folder. For most Unix-based systems these directories conventionally go in $XDG_CONFIG_HOME, which is usually ~/.local/share. This makes things neater for the user.

On Windows and macOS the variables XDG_CONFIG_HOME usually aren't defined, so you can use C:\Users\x\AppData\Roaming and ~/.local/share respectively (i.e., ~/.local/share does exist on macOS but isn't usually defined as XDG_CONFIG_HOME).

Steps to Reproduce

1. Run pio init
2. Observe new .platformio directory in $HOME

Start by tracing the directory selection used by pio init, including the core_dir configuration option and PLATFORMIO_CORE_DIR environment variable described in the issue. Done means the .platformio directory uses XDG_CONFIG_HOME where applicable and the stated macOS and Windows defaults otherwise, while existing overrides continue to work.