Build directory based on the build type

A known issue when PlatformIO rebuilds the project every time when the user flashes the board in the release mode and later starts debugging.

---


Replace
```python
BUILD_DIR = os.path.join("$PROJECT_BUILD_DIR", "$PIOENV")
```

to 

```python
BUILD_DIR = os.path.join("$PROJECT_BUILD_DIR", "$PIOENV", "$BUILD_TYPE")
```

