apm821xx: mbl_gen_hdd_img.sh: switch to /bin/sh

Nothing here needs bash.

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

target/linux/apm821xx/image/mbl_gen_hdd_img.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 
 set -x
 [ $# -eq 5 ] || {