init: add post mount user hooks

packages/sysutils/busybox/scripts/init

@@ -673,6 +673,10 @@
     wakeonlan
 
     mount_part "$boot" "/flash" "ro,noatime"
+
+    if [ -f /flash/post-flash.sh ] ; then
+      . /flash/post-flash.sh
+    fi
   }
 
   mount_storage() {
@@ -703,7 +707,12 @@
           disk="$target/$OVERLAY_DIR,$options"
         fi
       fi
+
+      if [ -f /flash/mount-storage.sh ] ; then
+        . /flash/mount-storage.sh
+      else
         mount_part "$disk" "/storage" "rw,noatime"
+      fi
     else
       # /storage should always be writable
       mount -t tmpfs none /storage