python,scons: Move readCommand and compareVersions into site_scons.

These functions are only used by scons, so it makes sense to move them
to site_scons/gem5_scons/util.py.

Change-Id: If2b3995f208cb71adf3c59aac4eabe378c47f94f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41599
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -63,7 +63,7 @@ Export('env')
 
 build_env = [(opt, env[opt]) for opt in export_vars]
 
-from m5.util import code_formatter, compareVersions, readCommand
+from m5.util import code_formatter
 
 ########################################################################
 # Code for adding source files of various types