firmware: start implementing close method

The close method is used to indicate something is done, for instance,
uploading a firmware image or tarball.

Change-Id: I9e972ea12282e700863d51db497c6294040cd2ca
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/image_handler.hpp b/image_handler.hpp
index 28675ce..ed3455b 100644
--- a/image_handler.hpp
+++ b/image_handler.hpp
@@ -26,6 +26,11 @@
     virtual bool open(const std::string& path) = 0;
 
     /**
+     * close the image.
+     */
+    virtual void close() = 0;
+
+    /**
      * write data to the staged file.
      *
      * @param[in] offset - 0-based offset into the file.