firmware: tie in call to data handler's open

A data handler can implement an open/close that is called during those
actions on the larger blob_id element.  This is meant to allow a
possible configure step for a driver on the BMC-side.

Change-Id: I62efa762d2efb8b2140b9856819554fbf577405a
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/test/firmware_writemeta_unittest.cpp b/test/firmware_writemeta_unittest.cpp
index b2338c1..13399fb 100644
--- a/test/firmware_writemeta_unittest.cpp
+++ b/test/firmware_writemeta_unittest.cpp
@@ -53,6 +53,7 @@
 
     auto handler = FirmwareBlobHandler::CreateFirmwareBlobHandler(blobs, data);
 
+    EXPECT_CALL(dataMock, open()).WillOnce(Return(true));
     EXPECT_CALL(imageMock2, open("asdf")).WillOnce(Return(true));
 
     EXPECT_TRUE(handler->open(