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_helper.hpp b/item_updater_helper.hpp
index 33b3cdb..a2d90b8 100644
--- a/item_updater_helper.hpp
+++ b/item_updater_helper.hpp
@@ -28,6 +28,13 @@
         // Empty
     }
 
+    /** @brief Set an environment variable to the specified value
+     *
+     * @param[in] entryId - The variable name
+     * @param[in] value - The variable value
+     */
+    void setEntry(const std::string& entryId, uint8_t value);
+
     /** @brief Clear an image with the entry id
      *
      * @param[in] entryId - The image entry id