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/Makefile.am b/Makefile.am
index a341233..c78a537 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -43,6 +43,7 @@
 	firmware_handler.cpp \
 	file_handler.cpp \
 	internal/sys.cpp \
+	prepare_systemd.cpp \
 	verify_systemd.cpp \
 	update_systemd.cpp