pylibfdt: Use Python2 explicitly

The pylibfdt code is written for Python2, not Python3.  So, it's safer to
explicitly request Python2 in our scripts and when checking pkg-config.

On Arch Linux at least, there isn't actually a plain "python" link, just
"python2" and "python3", so the current setup won't work at all.

According to https://www.python.org/dev/peps/pep-0394/

 using "python2"
should work, and is preferred.

Updating pylibfdt to work with Python3 would be nice, but is a problem for
another day.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>