Use /usr/bin/env in shebangs

Stop assuming that bash is in /bin, and that perl and python are in
/usr/bin, etc.
Instead, use env command to find these commands in the path.

tools/xorcheck.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 #  xorcheck.py - find xor values for 8-bit LRC
 #