Makefiles: Stop distributing files because building them requires Python.

Makefiles: Stop distributing files because building them requires Python.

A long time ago, the Open vSwitch build did not depend on Python (whereas
the runtime did), so the "make dist" based distribution included the
results of Python build tools.  Later, the build began using Python,
but the distribution still included some of those results, because no one
had gone to the trouble of changing them.  This commit changes the
Makefiles not to distribute Python-generated files but instead to just
generate them at build time.

Signed-off-by: Ben Pfaff <blp@nicira.com>