build: Explicitly use bash for shell scripts

VPP requires bash for all shell scripts. Align shebang lines in build
and test scripts to look up the location of bash rather than hard coding
'/bin/bash'.

Look up the location of bash for makefiles.

Type: improvement
Change-Id: I23b705d81d60389fa8af61c680cf0abd74f0ea24
Signed-off-by: Tom Jones <thj@freebsd.org>

build-root/scripts/set-rpath

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright (c) 2015 Cisco and/or its affiliates.
 # Licensed under the Apache License, Version 2.0 (the "License");