Closes #55 - Platform: Make board part optional

* making it optional was trivial, but support in PyNQ was not
* noticed that the board part of the ZedBoard and the support files for
the ZedBoard were used, this does not work (completely different board)
* had to pull the master XDC to find the clock pin and fix that
* Platforms can implement platform::create_clock_port to generate their
own clock ports
* plugin is used to generated the constraints "post-synth"
* Zedboard and PyNQ bitstreams build, but cannot be tested