scons: Move scons and python version checking into site_init.py.

scons: Move scons and python version checking into site_init.py.

In an effort to shrink and modularize the main scons files, this
change pulls the scons and python version checking code out of the
main file and into site_init.py which runs before the SConstruct
starts. This will be a place to put really generic code which has to
do with the very fundemental aspects of getting scons to work. Other
checks, like checks for particular tools or particular versions of
tools, will happen in other more specialized files.

Change-Id: Icd00ecadbe1141aef4dbadcf42d6ddef1f3a701f
Reviewed-on: https://gem5-review.googlesource.com/5561
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>