Added Banana Pi compatibility and support for Grove Chainable RGB Led

* fixed compile error in file SpiTransferBufferSpecs.cs (missing explicit cast at line 227)
* FileGpioConnection: replaced Raspberry specific platform check against a generic Unix check (this make it compatible with other /sys/class/gpio compatible platforms, eg. Banana Pi)
* Added Default driver fallback to FileGpioConnection on non-Raspberry Pi systems
* Added support for Grove Chainable RGB Led modules