Backend: use recovery umount for /vendor

* It is a *HACK*
* Avoid "Mount /cust (Device or resource busy) failed" error on orangefox which
  mounting /cust as /vendor and also runing a non-treble ROM.
* It is a Orangefox feature that mount both /vendor and /cust while runing a
  non-treble ROM (Obviously, there is a BUG that it shouldnt mount both but /cust
  only), so both of them have a same block. In this case, normal busybox unmount
  will only un-mount what we inputted. I guess that orangefox hacked umount command
  to make /cust is binding with /vendor.

(See comments in #41)

Signed-off-by: wloot <wlootlxt123@gmail.com>

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

@@ -49,7 +49,7 @@ unmount_all() {
     $bb umount /system_root;
   fi;
   $bb umount /system;
-  $bb umount /vendor;
+  umount /vendor;
   $bb umount /data) 2>/dev/null;
 }
 abort() {