scripts/gen_image_generic.sh: use /bin/sh

This has nothing bash specific.

Signed-off-by: Rosen Penev <[email protected]>

scripts/gen_image_generic.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 # Copyright (C) 2006-2012 OpenWrt.org
 set -e -x
 if [ $# -ne 5 ] && [ $# -ne 6 ]; then