ext,scons: Make kconfiglib available for use in SCons.

Change-Id: Ie9c902819ed0750a04f8318911fa914701c2a0fe
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56755
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>

site_scons/gem5_python_paths.py

@@ -45,6 +45,7 @@ root = fs.Dir('#')
 extra_python_nodes = [
     root.Dir('src').Dir('python').srcnode(), # gem5 includes
     root.Dir('ext').Dir('ply').srcnode(), # ply is used by several files
+    root.Dir('ext').Dir('Kconfiglib').Dir('import').srcnode(), # kconfiglib
 ]
 
 extra_python_paths = [ node.abspath for node in extra_python_nodes ]