PNOR Activation: Minor code-review fixes

This commit follows openbmc/openbmc#1716, addressing a pair of minor
issues raised in the code review process for that issue.

In order to shorten and clarify the function Activation::activation(),
much of the content is moved to a pair of private member functions. A
few other miscellaneous cosmetic changes are made for similar reasons.

Additionally, a function is added to unsubscribe activation objects from
dbus signals after the completion of the activation process.

Resolves openbmc/openbmc#1843

Change-Id: I815e3d70850aac1f870aa741b6415a7714696367
Signed-off-by: Michael Tritz <mtritz@us.ibm.com>
diff --git a/activation.hpp b/activation.hpp
index 7702c52..96517ea 100755
--- a/activation.hpp
+++ b/activation.hpp
@@ -250,6 +250,15 @@
          **/
         void subscribeToSystemdSignals();
 
+        /**
+         * @brief unsubscribe from the systemd signals
+         *
+         * Once the activation process has completed successfully, we can
+         * safely unsubscribe from systemd signals.
+         *
+         **/
+        void unsubscribeFromSystemdSignals();
+
         /** @brief Persistent sdbusplus DBus bus connection */
         sdbusplus::bus::bus& bus;
 
@@ -294,6 +303,12 @@
          * */
         void delete_() override;
 
+    private:
+        /** @brief Member function for clarity & brevity at activation start */
+        void startActivation();
+
+        /** @brief Member function for clarity & brevity at activation end */
+        void finishActivation();
 };
 
 } // namespace updater