Support asynchronous mode for I2C and SPI

[davetcc]

The two defines indicate if async mode is available, and when it is we should switch to use it:

• DEVICE_I2C_ASYNCH indicates that I2C can operate asynchronously
• DEVICE_SPI_ASYNCH indicates that SPI can operate asynchronously

In the event that the bus was busy, which is detected by the return value from transfer indicating the bus was already locked, we would provide a function pointer that should be executed when the bus is busy, by default it would be taskManager.yieldForMicros(0);, but in the event the graphics library was not running in the main thread, another suitable yield function could be used.