CommonInstaller: another try for A/B device support

CommonInstaller

@@ -76,7 +76,7 @@ grep_prop() {
 	REGEX="${1}"
 	shift
 	FILES="${@}"
-	[ -z "${@}" ] && FILES='/system/build.prop'
+	[ -z "${@}" ] && FILES='/system/build.prop /system_root/build.prop'
 	sed -n "s/^${REGEX}=//p" ${FILES} | \
 		head -n 1
 }
@@ -196,21 +196,20 @@ mount_partitions () {
 
 	${BOOTMODE} || mount -o bind /dev/urandom /dev/random
 
-	[ ! -f /system/build.prop ] && mount -o rw /system 2>/dev/null
+	! is_mounted /system && mount -o rw /system 2>/dev/null
 
 	if [ ! -f /system/build.prop ]; then
 		SYSTEMBLOCK=$(find /dev/block -iname system${SLOT} | head -n 1)
 		mount -t ext4 -o rw $SYSTEMBLOCK /system
 	fi
 
+	[ -f /system/build.prop ] || is_mounted /system || error "failed to mount /system"
+
 	if [ -f /system/init ]; then
 		mkdir /system_root 2>/dev/null
 		mount --move /system /system_root
 		mount -o bind /system_root/system /system
 	fi
-
-	[ ! -f /system/build.prop ] && error "failed to mount /system (unsupported A/B device?)"
-
 }
 
 detect_mode () {