Python3 Migration: make with PY_VERSION to handle Python 2&3 switching

Let make handle Python 2&3 switching with the PY_VERSION flag of which
the default value is set to python2.

BUG=b:169724328
TEST=Conduct the following tests
python3 -m compileall -q .
python2 -m compileall -q .
python3 -m py_compile *.py
python2 -m py_compile *.py
Bluetooth AVL tests

Change-Id: Iefa8f8ab6c9d3d979d55fef29015ba69b88e6e59
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/chameleon/+/2497730
Reviewed-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
Tested-by: Shyh-In Hwang <josephsih@chromium.org>
Commit-Queue: Shyh-In Hwang <josephsih@chromium.org>
3 files changed