avoid using system("mkdir") in favor of Python code

[Roland Haas]

This tends to make things more robust wrt to error checking and unexpected spaces in file names.

Pull request is here: https://bitbucket.org/simfactory/simfactory2/pull-requests/51/avoid-using-system-mkdir-in-favor-of

[Roland Haas]

Please review.

[Roland Haas]

Applied as git hash 5635c7b7 "sim-build: use os.mkdir instead of system("mkdir")" of simfactory2