init: use '/system' as the system mount point for force_normal_boot

We need to keep the right mount points in the fstab for other
processes to access (/ for system-as-root or /system otherwise).
Force_normal_boot devices are defacto not system-as-root,
since they're booting through the recovery ramdisk and therefore must
use /system as the system mount point.

Bug: 114062208
Test: boot pixel from recovery with force_normal_boot
Change-Id: Ib0e4e4f8a29299ecdcd35e8f1415551c4c9745ad