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>

scons: Move python path management out of the SConstruct.

Make site_init.py manage sys.path, and the "default" tool set
PYTHONPATH on any environment that's created. The paths to add are
tracked in a common gem5_python_paths.py.

Change-Id: I3387d4394d47a2f9c83322644cfd05909c6890fa
Reviewed-on: https://gem5-review.googlesource.com/5564
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

scons: Pull style hook management out of the main SConstruct.

Put the code which supports style hooks for mercurial and git into two
scons "tools".

Change-Id: I3ffed85a177be4f9e458fff7b1cf16a3a479914e
Reviewed-on: https://gem5-review.googlesource.com/5563
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

scons: Override the "default" tool to set up the default env.

This imports various environment variables into the scons environment,
and sets some general properties on it. These are basically just copied
directly from the SConstruct and have the same behavior here.

gem5_tool_list will be used later on to add scons "tools" which should
be automatically added to new Environment objects.

Change-Id: Ib255955090c7b1e1cb80c703c18a9c867fcf1c9e
Reviewed-on: https://gem5-review.googlesource.com/5562
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>