Build pylibfdt as part of the normal build process

If swig and the Python are available, build pylibfdt automatically.
Adjust the tests to run Python tests too in this case.

Signed-off-by: Simon Glass <sjg@chromium.org>
[dwg: Make error message clearer that missing swig or python-dev isn't
 fatal to the whole build]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>