McuSupportPlugin: env var as fallback default path

In Qt for Mcus 2.0, the environment variables are not used any more, using cmake variables instead.

However, there is still a use case for using those environment variables for pre-populating the cmake variables.

The idea here is to restore the env var within the json kit description.

Also, the version checker information should be part of the block that is being checked, in the json.

Example:

  "boardSdk": {
    "cmakeCacheEntries": [
      {
        "id": "ST_SDK_DIR",
        "description": "Board SDK for STM32F769I-Discovery",
        "cmakeOptionName": "QUL_BOARD_SDK_DIR",
        "type": "path",
        "optional": false,
        "envVar": "STM32Cube_FW_F7_SDK_PATH", 
        "versions": [ "1.16.0" ]
      }
    ]
  },

In this example, no default path is given. But the plugin can check if one is defined as the given environment variable and use that for pre-populating the cmake variable.

Also, the field "versions", which was part of "boardSdk", is part of the variable block. If there are other additional cmakeCacheEntries, this resolves the possible ambiguity.