treewide: use /usr/bin/env where appropriate

Some Unix systems (GuixSD, NixOS) do not install programs like
Bash and Python to /usr/bin, and /usr/bin/env has to be used to
locate these instead.

Change-Id: I7546bcb881c532adc984577ecb0ee2ec4f2efe00
Signed-off-by: Yegor Timoshenko <yegortimoshenko@riseup.net>
Reviewed-on: https://review.coreboot.org/28953
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/ipqheader/mbn_tools.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 #===============================================================================
 #
 # MBN TOOLS