xcompile: use bash

I don't think all /bin/sh implement all features used
in xcompile.

Change-Id: Ida2a166242201ed0221316b123888127c83bf3c1
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/5740
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/xcompile/xcompile

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # This file is part of the coreboot project.
 #