add Vivado support

[jschlatow]

This commit adds support for recreating Xilinx Vivado projects and generating bitstreams. For this purpose, Goa detects the presence of a vivado.tcl file in the src/ directory. This file must be created in Vivado with the following command:

write_project_tcl -paths_relative_to /path/to/workspace/dir vivado.tcl

Furthermore, the required source files (as referenced in the checkRequiredFiles procedure in the tcl file) must be manually copied into the src/ directory.

Note that the Vivado tools must have been installed and sourced.

[nfeske]

Merged to master. It would nice to document this feature down the road, e.g., via goa help vivado.