test: vboot: explicitly request bash

vboot_test.sh uses Bashisms. Explicitly use #!/bin/bash so the script
doesn't fail if /bin/sh isn't Bash.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

test/vboot/vboot_test.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright (c) 2013, Google Inc.
 #