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>
diff --git a/extras/vagrant/update.sh b/extras/vagrant/update.sh
index 3fb456b..e56e6ba 100755
--- a/extras/vagrant/update.sh
+++ b/extras/vagrant/update.sh
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Make sure that we get the hugepages we need on provision boot
 # Note: The package install should take care of this at the end