releasetools: Move system mount handling to backuptool

* This allows the ROM to be flashed on any recovery, whether it mounts
  system to /system, /system_root or /mnt/system like Q recovery

* Remove any type of system mount with static paths from updater-script
  to figure out the path dinamically through backuptool

Change-Id: I7b33726305cb6f3eec146d26135f0081002a25dc