scripts/symlink-tree.sh: use /bin/sh

This uses nothing bash specific.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

scripts/symlink-tree.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # Create a new openwrt tree with symlinks pointing at the current tree
 # Usage: ./scripts/symlink-tree.sh <destination>