vivado xsim xci compatibility. compatible with vivado synthesis.

[skokvermon]

This adds possibility of simulating vivado XCI in xsim.
Compatible with vivado synthesis XCI support.
Automatically calls vivado to generate code from the makefile

needs a new parameter: part which select xilinx part number

Also added a side feature in the makefile to enable vcd output.
the default makefile run is now selectable with new parameter: default_run

fixes bug #344

example

tools:
  xsim:
    default_run: run-vcd
    part: xc7a100tcsg324-2

[olofk]

I think this might be better to handle with a generator so that all simulators can benefit from this. Also, the xsim backend currently only implements the old Tool API. We should focus on porting it to the new Flow API.