patman: Don't run patman when it is imported as a module

Commit 488d19cb (patman: add distutils based installer) has the side effect
of making patman run twice with each invocation. Fix this by checking for
'main program' invocation in patman.py. This is good practice in any case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Chris Packham <judge.packham@gmail.com>