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>

ext/libelf/SConscript

@@ -32,8 +32,6 @@ import os, subprocess
 
 Import('main')
 
-from m5.util import compareVersions
-
 elf_files = []
 def ElfFile(filename):
     elf_files.append(File(filename))