rb532: gen_image.sh: use /bin/sh

Nothing here needs bash.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
[slightly adjust commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>

target/linux/rb532/image/gen_image.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 OUTPUT="$1"
 KERNELSIZE="$2"
 KERNELIMAGE="$3"