build: fixup python SetupHostCommand to use python2

build: fixup python SetupHostCommand to use python2

Here is a way to break your build env without this patch:
1) have python point to python3, and no python2
2) start the build, SetupHostCommand will create a symlink
./staging_dir/host/bin/python -> /usr/bin/python
3) build fails on scons because it can't find any python2
4) install python2 and restart the build
5) the build fails on wireless-regdb compile because python is python3 instead of python

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>