firmware: add write method

The write() method is how the different firmware handlers will receive
data from the update process.

Change-Id: I550a25f69272bae917309956fef798bc3d4eed2e
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/test/image_mock.hpp b/test/image_mock.hpp
index 4e615b1..6fbd87d 100644
--- a/test/image_mock.hpp
+++ b/test/image_mock.hpp
@@ -13,6 +13,7 @@
     virtual ~ImageHandlerMock() = default;
 
     MOCK_METHOD1(open, bool(const std::string&));
+    MOCK_METHOD2(write, bool(std::uint32_t, const std::vector<std::uint8_t>&));
 };
 
 } // namespace blobs