build: Allow mounting system properly for A-only system-as-root devices

 * By AOSP build system and recovery design, A-only system-as-root
   image should be mounted at /system_root and /system is a symlink
   to /system_root/system.
 * Still allow mounting system to /system for system-as-root using a variable for devices that eventually need this

Change-Id: I4184c572b25faa8d1990e32f625fa814cba6690f