move systemd subscribe to util function

This code is replicated in multiple source files in this repo, put it in
the util repo so everyone can call it in one place.

Tested:
- Verified systemd signals seen and system boots

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I8bf0343c79012cc91dea033038894a011fd37ef5
diff --git a/host_state_manager.hpp b/host_state_manager.hpp
index 8e60130..fc80da8 100644
--- a/host_state_manager.hpp
+++ b/host_state_manager.hpp
@@ -3,6 +3,7 @@
 #include "config.h"
 
 #include "settings.hpp"
+#include "utils.hpp"
 #include "xyz/openbmc_project/State/Host/server.hpp"
 
 #include <cereal/access.hpp>
@@ -69,7 +70,7 @@
         settings(bus, id), id(id)
     {
         // Enable systemd signals
-        subscribeToSystemdSignals();
+        utils::subscribeToSystemdSignals(bus);
 
         // create map of target name base on host id
         createSystemdTargetMaps();
@@ -152,15 +153,6 @@
 
   private:
     /**
-     * @brief subscribe to the systemd signals
-     *
-     * This object needs to capture when it's systemd targets complete
-     * so it can keep it's state updated
-     *
-     **/
-    void subscribeToSystemdSignals();
-
-    /**
      * @brief Determine initial host state and set internally
      *
      * @return Will throw exceptions on failure