Add support for vivado ip cache

[alessandrothea]

Vivado is capable of caching out-of-context run results outside projects and re-use them, potentially saving a time and resources. Here's an example of the involved tcl commands

config_ip_cache -import_from_project -use_cache_location /home/ale/devel/dune/dataflow_ips
update_ip_catalog

In terms of ipbb integration, the question to answer is how to best handle the cache

• locally (within the project), in the work area, outside the work area
• how to enable caching, flush the cache and in general make sure that synthesis results are always consistent

[alessandrothea]

Actually done and merged back in dev/master