Backend: fix getting parent directory on some devices

/tmp/updater: line 33: cd: /tmp/anykernel/..: No such file or directory

Closes #41

META-INF/com/google/android/update-binary

@@ -30,7 +30,7 @@ int2ver() {
   fi;
 }
 cleanup() {
-  cd $home/..;
+  cd $(dirname $home);
   rm -rf $home;
 }
 debugging() {