Use portable shebangs (/bin/bash -> /usr/bin/env bash)

"/usr/bin/env bash" is used everywhere else in host tools. These are the
only remaining offenders.

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/atmel/flasher.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 BUILDIR=$1
 TTY=$2

board/wandboard/post-image.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 GENIMAGE_CFG="board/wandboard/genimage.cfg"
 GENIMAGE_TMP="${BUILD_DIR}/genimage.tmp"