backuptool: Execute check_blacklist from the current directory

* For some odd reasons executing `cd /system/addon.d` makes the system
   hang and unmount error:
     umount: /system_root: Device or resource busy
 * Don't change directory to not allow the system partition hang

Change-Id: I3d30bdc59c2f05d16823e99046c1dce2e1e6eb73

prebuilt/common/bin/backuptool.sh

@@ -59,8 +59,7 @@ return 1
 check_blacklist() {
   if [ -f $S/addon.d/blacklist -a -d /$1/addon.d/ ]; then
       ## Discard any known bad backup scripts
-      cd /$1/addon.d/
-      for f in *sh; do
+      for f in /$1/addon.d/*sh; do
           [ -f $f ] || continue
           s=$(md5sum $f | cut -c-32)
           grep -q $s $S/addon.d/blacklist && rm -f $f