Move setting the priority U-Boot variable to ubi

The service that sets the priority value is unique to the ubi
layout because there is only one version at a time on the
static layout.
Move the calling of the service to the ubi implementation.

Tested:
- Witherspoon: Priorities are still set.
- Romulus: The BMC.Updater app does not core dump with the
latest sdbusplus changes due to calling a non-existent service.

Change-Id: Ica1c68f00d5cb43c51ee09c5a3851613edf941d5
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/item_updater.hpp b/item_updater.hpp
index 330ec82..ec7aee0 100644
--- a/item_updater.hpp
+++ b/item_updater.hpp
@@ -61,6 +61,15 @@
         emit_object_added();
     };
 
+    /** @brief Save priority value to persistent storage (flash and optionally
+     *  a U-Boot environment variable)
+     *
+     *  @param[in] versionId - The Id of the version
+     *  @param[in] value - The priority value
+     *  @return None
+     */
+    void savePriority(const std::string& versionId, uint8_t value);
+
     /** @brief Sets the given priority free by incrementing
      *  any existing priority with the same value by 1
      *