bmc: add preparation step before data is received

Add a preparation systemd trigger event before data is received.  On
systems under memory pressure, this'll trigger a service that can do
things like flush caches.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I175177f4a91b58d9f163098572a9d2614002b718
diff --git a/test/firmware_state_notyetstarted_unittest.cpp b/test/firmware_state_notyetstarted_unittest.cpp
index 528e47f..54ab7f8 100644
--- a/test/firmware_state_notyetstarted_unittest.cpp
+++ b/test/firmware_state_notyetstarted_unittest.cpp
@@ -93,6 +93,7 @@
 TEST_F(FirmwareHandlerNotYetStartedTest, OpenStaticImageFileVerifyStateChange)
 {
     EXPECT_CALL(imageMock, open(staticLayoutBlobId)).WillOnce(Return(true));
+    EXPECT_CALL(*prepareMockPtr, trigger()).WillOnce(Return(true));
 
     EXPECT_TRUE(handler->open(session, flags, staticLayoutBlobId));
 
@@ -104,6 +105,7 @@
 TEST_F(FirmwareHandlerNotYetStartedTest, OpenHashFileVerifyStateChange)
 {
     EXPECT_CALL(imageMock, open(hashBlobId)).WillOnce(Return(true));
+    EXPECT_CALL(*prepareMockPtr, trigger()).WillOnce(Return(true));
 
     EXPECT_TRUE(handler->open(session, flags, hashBlobId));