mediatek: gen_mt7623_emmc_img.sh: use /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/mediatek/image/gen_mt7623_emmc_img.sh

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/bin/sh
 
 OUTPUT_FILE=$1
 KERNEL_FILE=$2