nrf5/sdk: Adding macro based ringbuffer written by Philip Thrasher. source: https://github.com/pthrasher/c-generic-ring-buffer/blob/master/ringbuffer.h. Copyright noticed copied into the file, and file reviewed by Philip.
nrf5/sdk: Adding macro based ringbuffer written by Philip Thrasher. source: https://github.com/pthrasher/c-generic-ring-buffer/blob/master/ringbuffer.h. Copyright noticed copied into the file, and file reviewed by Philip.

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