Sync PSU images on service startup

On service startup, it shall check the PSU images, find a latest
version, and update to the PSUs that are not with this version.

Tested: With dummy image and service, test on Witherspoon with 2
        different running PSU software:
* When startup without stored image, the serive tries to update an older
PSU but it does not have PSU image, so it's skipped with below journal
log:
    Automatically update PSU
    No image for the activation, skipped
* When startup with an older image stored in BMC, it behaves the same as
above.
* When startup with a new image stored in BMC, it updates the PSUs, with
below example journal log:
    Automatically update PSU
    Starting Update PSU
    /xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply0
    /var/lib/obmc/psu/2B1D...
    Started Update PSU
    /xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply0
    /var/lib/obmc/psu/2B1D.
    Starting Update PSU
    /xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply1
    /var/lib/obmc/psu/2B1D...
    Started Update PSU
    /xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply1
    /var/lib/obmc/psu/2B1D.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I6d676c5a4441685fb2b5920455f439c00f6097af
diff --git a/src/activation.hpp b/src/activation.hpp
index be373c5..669686e 100644
--- a/src/activation.hpp
+++ b/src/activation.hpp
@@ -176,6 +176,12 @@
     RequestedActivations
         requestedActivation(RequestedActivations value) override;
 
+    /** @brief Get the object path */
+    const std::string& getObjectPath() const
+    {
+        return objPath;
+    }
+
     /** @brief Get the version ID */
     const std::string& getVersionId() const
     {