ot::Spinel::Buffer::OutFrameRead() method

Reads and copies bytes from the current output frame into a given buffer. The NCP buffer maintains a read offset for the current output frame being read. This method attempts to read the given number of bytes (@p aReadLength) from the current frame and copies the bytes into the given data buffer (@p aDataBuffer). It also moves the read offset forward accordingly. If there are fewer bytes remaining in current frame than the requested @p aReadLength, the available bytes are read/copied. This methods returns the number of bytes read from frame and copied into @p aDataBuffer.