Refactoring: Privatize ring buffer config header

Refactoring: Privatize ring buffer config header

The public header ringbuffer-config.h contains two distinct sets of APIs
and structure declarations:

- Ring buffer configuration, which should become private (moved to
  libringbuffer/ringbuffer-config.h),
- Ring buffer context for use when writing an event record into the
  ring buffer. This part is moved to a new "ringbuffer-context.h" public
  header.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I07dae33ab5e3925dea0158058d484c49292a0bb0