scons: Build error introduced by site_tools/default.py

One of the latest commits affecting the build system introduced a change
in the environment setting: rather than using the ENV dictionary,
environment key-val pairs were saved directly into the class. This
patch reverts that behaviour.

Change-Id: If206ba0a34ac8c35695a493cb90654b7ec81971b
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/5941
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

site_scons/site_tools/default.py

@@ -64,7 +64,7 @@ def common_config(env):
     for key,val in sorted(os.environ.iteritems()):
         if key in use_vars or \
                 any([key.startswith(prefix) for prefix in use_prefixes]):
-            env[key] = val
+            env['ENV'][key] = val
 
     # Tell scons to avoid implicit command dependencies to avoid issues
     # with the param wrappes being compiled twice (see