boards_dir
Type: DirPath | Multiple: No | Default: “<Project>/boards”
The location of project-specific board definitions. Each project may
choose a suitable directory name. The default value is boards,
meaning a “boards” directory located in the root of the project.
By default, PlatformIO looks for boards in this order:
Project boards_dir (as defined by this setting)
Global core_dir/boards
Development platform core_dir/platforms/*/boards.
This option can also be configured by the global environment variable
PLATFORMIO_BOARDS_DIR.