Revert "Don't mount SAR system to /system"

* This is horribly broken

This reverts commit 7c6536c1e79fc5b48d057fdc179ee794ad6a8ad6.

build/meta/com/google/android/update-binary

@@ -48,9 +48,8 @@ if [ "$SYSTEMASROOT" == "true" ]; then
     TARGETSYSTEM=/dev/block/bootdevice/by-name/system
   fi
   mkdir -p /system_root
-  if mount -o rw $TARGETSYSTEM /system_root; then
+  if mount -o rw $TARGETSYSTEM /system_root && mount /system_root/system /system; then
     ui_print "/system mounted"
-    SYSTEM=/system_root/system
   else
     ui_print "Could not mount /system! Aborting"
     exit 1
@@ -58,7 +57,6 @@ if [ "$SYSTEMASROOT" == "true" ]; then
 else
   if mount /system; then
     ui_print "/system mounted"
-    SYSTEM=/system
   else
     ui_print "Could not mount /system! Aborting"
     exit 1
@@ -133,9 +131,9 @@ for dirs in $DIRS; do
   done
 done
 ui_print "Copying files"
-exec_util "cp --preserve=a -r ./* $SYSTEM/"
+exec_util "cp --preserve=a -r ./* /system/"
 if [ -e priv-app/SetupWizard ] ; then
-    exec_util "rm -rf $SYSTEM/priv-app/Provision/"
+    exec_util "rm -rf /system/priv-app/Provision/"
 fi
 ui_print "Cleaning up files"
 cd ../