scons: Move Transform and termcap functionality into their own files.

Change-Id: Ica08e93f3873a7eafd02fe7d44c3bdbf0ce7f6b7
Reviewed-on: https://gem5-review.googlesource.com/5565
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>