backuptool: Run check_{black, white}list with proper system path

 * If any of these two function gets run on a recovery mounting system
   to /system, /system/addon.d won't exist while /system/system/addon.d will.
 * Run the functions with $S as argument to make this work correctly

Change-Id: I02e7b91429a9e74d28bdb77e56955dad97ca75ac