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.hpp b/bmc/update_systemd.hpp
index fe0ab33..a8dd8fc 100644
--- a/bmc/update_systemd.hpp
+++ b/bmc/update_systemd.hpp
@@ -36,6 +36,8 @@
     void abort() override;
     ActionStatus status() override;
 
+    const std::string getMode() const;
+
   private:
     sdbusplus::bus::bus bus;
     const std::string target;