scripts/install-buildtools: refactor for Python 3.4

scripts/install-buildtools: refactor for Python 3.4

Our least common denominator supported distro is debian-8
which has python 3.4. The whole point of the install-buildtools
script is to make it easier on the user to install buildtools
tarball. So it needs to run on Python 3.4.

The way we checked if the install was successful in the prior
version of the script was not workable in python 3.4. Since
the environment-setup-... script is currently just exporting
environment variables, use os.environ to do the equivalent from
values gleaned via regex from the environment-setup-... file.

Corrected a couple minor whitespace errors

NOTE: License changed to GPL-2.0-only due to inclusion of code
copied directly from bitbake/lib/bb/utils.py. This avoids the
need to depend on bitbake, which is now Python 3.5+ only.

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>