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>

site_scons/site_tools/git.py

@@ -42,7 +42,6 @@ import os
 import sys
 
 import gem5_scons.util
-from m5.util import readCommand
 
 git_style_message = """
 You're missing the gem5 style or commit message hook. These hooks help
@@ -52,7 +51,7 @@ Press enter to continue, or ctrl-c to abort: """
 
 def install_style_hooks(env):
     try:
-        gitdir = env.Dir(readCommand(
+        gitdir = env.Dir(gem5_scons.util.readCommand(
             ["git", "rev-parse", "--git-dir"]).strip("\n"))
     except Exception as e:
         print("Warning: Failed to find git repo directory: %s" % e)