feat[vivado]: fix for #426

[shareefj]

This commit updates tools/vivado.py to allow the user to skip the link_design step normally used after reading in a toplevel netlist.

The use case here is when an EDIF netlist, or any other netlist, is read in as a submodule and is not the toplevel. In this case the link_design step will fail as Vivado is expecting a netlist with the toplevel name.

In order to bypass the link_design step, use the no_link_design tag when specifying the netlist in your Core file:

filesets:
netlist:
files:
- cute_a7_core/cute_a7_core.edf: { file_type: edif, tags: [no_link_design] }