AK3+Backend: mount and output fixes

- fix case where TWRP has /system_root mountpoint but ROM is non-SAR
- fix regression in ui_print when final character is \

tools/ak3-core.sh

@@ -15,7 +15,8 @@ split_img=$home/split_img;
 # ui_print "<text>" [...]
 ui_print() {
   until [ ! "$1" ]; do
-    echo -e "ui_print $1\nui_print" > /proc/self/fd/$OUTFD;
+    echo -e "ui_print $1
+      ui_print" > /proc/self/fd/$OUTFD;
     shift;
   done;
 }