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