ot::Spinel::Buffer::OutFrameBegin() method

Begins/prepares an output frame to be read from the frame buffer if there is no current active output frame, or resets the read offset if there is a current active output frame. The NCP buffer maintains a read offset for the current frame being read. Before reading any bytes from the frame this method should be called to prepare the frame and set the read offset. If part or even all of current frame has been read, a sub-sequent call to this method will reset the read offset back to beginning of current output frame (note that the current output frame will remain unchanged even in case where a higher priority frame was written to buffer while reading current output frame). A prepared output frame will stay active as current output frame until it is explicitly removed using `OutFrameRemove()`.