patman: Tidy up the start-up code

patman: Tidy up the start-up code

Much of this was written before the other Python tools and they have
evolved. Make a few updates:

- Rather than calling sys.exit(), return the exit code from the control
  module and use it in __main__
- Set up tout as it is used in some places
- We now have quite a few tests, so update the comment about that

Signed-off-by: Simon Glass <sjg@chromium.org>