Allow custom variants directory

[loick111]

The goal is to allow custom configuration for variants_dir to define our own board variant directly in the project.

With this functionnality, we are now allowed to define in our projects a custom board AND a custom variant.
https://docs.platformio.org/en/latest/platforms/creating_board.html#custom-embedded-boards

Here is an example of how to define a custom board with custom variant:

my_project
├── boards
│   └── custom_esp32dev.json
└── variants
    └── custom-esp32dev
        └── pins_arduino.h

custom_esp32dev.json

{
  "build": {
    "arduino":{
      "ldscript": "esp32_out.ld"
    },
    "core": "esp32",
    "extra_flags": "-DARDUINO_ESP32_DEV",
    "f_cpu": "240000000L",
    "f_flash": "40000000L",
    "flash_mode": "dio",
    "mcu": "esp32",
    "variants_dir": "variants",
    "variant": "custom-esp32dev"
  },
  "connectivity": [
    "wifi",
    "bluetooth",
    "ethernet",
    "can"
  ],
  "debug": {
    "openocd_board": "esp-wroom-32.cfg"
  },
  "frameworks": [
    "arduino",
    "espidf"
  ],
  "name": "My Custom Espressif ESP32 Dev Module",
  "upload": {
    "flash_size": "4MB",
    "maximum_ram_size": 327680,
    "maximum_size": 4194304,
    "require_upload_port": true,
    "speed": 460800
  },
  "url": "https://en.wikipedia.org/wiki/ESP32",
  "vendor": "Espressif"
}

[me-no-dev]

Hey thanks for this :) Could I bother you to add the changes over to the idf-release/v4.2 branch as well?

https://github.com/espressif/arduino-esp32/blob/idf-release/v4.2/tools/platformio-build-esp32.py

https://github.com/espressif/arduino-esp32/blob/idf-release/v4.2/tools/platformio-build-esp32s2.py

[loick111]

Hi! Thanks for merging this PR really quickly! :)
Yes of course, I will create a pull request on this branch.

[me-no-dev]

thanks!