Add camera exposure setter to v4l2 reader

For rapid react we will use low exposure to focus on the reflected
light.

Signed-off-by: milind-u <milind.upadhyay@gmail.com>
Change-Id: Id1d4b574b13672229f6054e115870b239d21bc5b
diff --git a/y2020/vision/v4l2_reader.h b/y2020/vision/v4l2_reader.h
index 04548d6..b9b3ce4 100644
--- a/y2020/vision/v4l2_reader.h
+++ b/y2020/vision/v4l2_reader.h
@@ -43,6 +43,13 @@
                                              buffer->message_offset);
   }
 
+  // Sets the exposure duration of the camera. duration is the number of 100
+  // microsecond units.
+  void SetExposure(size_t duration);
+
+  // Switches from manual to auto exposure.
+  void UseAutoExposure();
+
  private:
   static constexpr int kNumberBuffers = 16;