Add updateable association

Add 'updateable' association to the functional PSU firmware.
This 'updateable' association can be used to mark all the
firmware components which can be programmable from BMC
interfaces like Redfish.

Resolves openbmc/phosphor-psu-code-mgmt#2

Tested: Verify unit tests success;
        Verify the updateable associations are created for PSU firmware
        objects on Witherspoon, and the Redfish has "Updateable": true
        for the PSU images.

Signed-off-by: Lei YU <mine260309@gmail.com>
Change-Id: I579121587b0d2c14f08beceaa68e380eaf1eefd9
diff --git a/src/association_interface.hpp b/src/association_interface.hpp
index e3d653c..2969719 100644
--- a/src/association_interface.hpp
+++ b/src/association_interface.hpp
@@ -21,6 +21,13 @@
      */
     virtual void addFunctionalAssociation(const std::string& path) = 0;
 
+    /** @brief Add the updateable association to the
+     *  "running" PSU software image
+     *
+     * @param[in]  path - The path to create the association.
+     */
+    virtual void addUpdateableAssociation(const std::string& path) = 0;
+
     /** @brief Remove the associations from the provided software image path
      *
      * @param[in]  path - The path to remove the association from.