bmc: json configuration: update: add mode parameter

Add the mode parameter as an optional configuration option for the
update systemd approach.  Previously this was hard-coded as "replace"
but now the user can specify something else.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: I2bcb61bd9ac5733df6da70f901516d26bd31e2a9
diff --git a/bmc/update_systemd.cpp b/bmc/update_systemd.cpp
index 92c97c8..afd4316 100644
--- a/bmc/update_systemd.cpp
+++ b/bmc/update_systemd.cpp
@@ -69,4 +69,9 @@
     return ActionStatus::running;
 }
 
+const std::string SystemdUpdateMechanism::getMode() const
+{
+    return mode;
+}
+
 } // namespace ipmi_flash