bugfix: bmc: update service requires mode

Systemd startunit requires a job-mode.  Default it to "replace."  In a
later patchset, the update systemd and verify system will collapse into
one object, but this fix applies to the current implementation.

Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ia6465abac9979501ea7149933394d8c452b2f315
diff --git a/bmc/buildjson.cpp b/bmc/buildjson.cpp
index a359add..aec9a09 100644
--- a/bmc/buildjson.cpp
+++ b/bmc/buildjson.cpp
@@ -113,7 +113,7 @@
             {
                 const auto& unit = update.at("unit");
                 pack->update = SystemdUpdateMechanism::CreateSystemdUpdate(
-                    sdbusplus::bus::new_default(), unit);
+                    sdbusplus::bus::new_default(), unit, "replace");
             }
             else
             {