tools: added cloud.py, which starts an EC2 instance with the Xilinx tools available.

This works towards the goal of CI in EC2.

The script is currently just a shell with placeholders for the parts of the script that actually do the work of building and the like.
Should have a header comment, I'd recommend the following...

```python
#!/usr/bin/env python
# -*- coding: utf-8 -*-
# vim: set ts=4 sw=4 et sts=4 ai:
```