clang-tidy: Enable readability-convert-member-functions-to-static

This check finds non-static member functions that can be made
static because the functions don’t use this.
This check also triggers readability-static-accessed-through
-instance check as we are trying to access a static member
function through an instance.

Change-Id: Ia0867db019db8e40e710bdd4025e030061f7a992
Signed-off-by: Pavithra Barithaya <pavithrabarithaya07@gmail.com>
diff --git a/item_updater_helper.hpp b/item_updater_helper.hpp
index c0584a2..2b10e8f 100644
--- a/item_updater_helper.hpp
+++ b/item_updater_helper.hpp
@@ -47,7 +47,7 @@
     void cleanup();
 
     /** @brief Do factory reset */
-    void factoryReset();
+    static void factoryReset();
 
     /** @brief Remove the image with the flash id
      *