scripts/coreos-bblayers-envsub: fixed path of shebang

there might be OS where usr merge is not included and thus
/bin/env is not available as it resides in /usr/bin/env.

Signed-off-by: Marc Mattmueller <marc.mattmueller@netmodule.com>

scripts/coreos-bblayers-envsub

@@ -1,4 +1,4 @@
-#!/bin/env bash
+#!/usr/bin/env bash
 
 usage()
 {