Add global support for custom boards variants directory.

[4project-co-il]

The scripts will search for the "variants_dir" directory specified in custom board ".json" file in:

• project root directory
• core_dir
• framework installed "variants"

Tested cases with project and core_dir. When both locations were not present, the script defaulted to ~/.platformio/packages/framework-arduino-samd/variants as expected

[valeros]

Hi @4project-co-il, thanks for the PR, although I had a slightly more generic implementation in mind. The idea was to allow a developer to use an arbitrary path, including PlatformIO-specific directories like $PROJECT_CORE_DIR.

Do you mind if I push additional changes to your PR?

[4project-co-il]

Feel free to change whatever needed.

I saw the $PROJECT_CORE_DIR. Couldn't find what was the intention of that directive.

Everything is possible with the right configuration.
I want to easy on the end customer that should do that configuration.

Right now in order to use a custom board, the end user should copy the boards directory to the core_dir, so the board will appear in the UI during project creation.
After the project is created, the variants directory should be copied to the project root.

I want to minimize the process and remove the second step.
The manufacturer can provide the boards and variants directory and instruct the customer to copy both those directories to the `.platformio' and that's it. No need for the second step. No need to define anything else.
Just an extention to the pre-installed directories in the arduino framework.

[valeros]

Thanks, merged to the dev branch. Now the following options are supported:

[env:zero]
platform = atmelsam
framework = arduino
board = zero
board_build.variants_dir = project_variants_dir
; board_build.variants_dir = ../../relative_variants_dir
; board_build.variants_dir = ${platformio.core_dir}/core_variants_dir
; board_build.variants_dir = $PROJECT_CORE_DIR/core_variants_dir
; board_build.variants_dir = /arbitrary/path/in/system/variants_dir