scripts: Call out python2 explicitly

Many systems these days have both python2 and python3 installed.  The
unversioned executable `python` may point to either version.  Per
https://www.python.org/dev/peps/pep-0394/

 , scripts that need Python 2
should explicitly call this out in the shebang line.

These build scripts are both specific to python 2, so call this out in
the shebang line.

Change-Id: I11f70c341e484254dc1679db65c2ed5ac61e510f
Signed-off-by: David Brown <davidb@codeaurora.org>