feat[vivado]: fix for #426

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] }

edalize/tools/vivado.py

@@ -105,6 +105,7 @@ def setup(self, edam):
         has_xci = False
         unused_files = []
         bd_files = []
+        netlist_flow = False
 
         dep_files = []
         for f in self.files:
@@ -119,6 +120,8 @@ def setup(self, edam):
             elif file_type == "edif":
                 cmd = "read_edif"
                 edif_files.append(f["name"])
+                if not 'no_link_design' in f.get('tags', []):
+                    netlist_flow = True
             elif file_type.startswith("vhdlSource"):
                 cmd = "read_vhdl"
                 if file_type == "vhdlSource-2008":
@@ -176,7 +179,7 @@ def setup(self, edam):
             "vlogparam": self.vlogparam,
             "vlogdefine": self.vlogdefine,
             "generic": self.generic,
-            "netlist_flow": bool(edif_files),
+            "netlist_flow": netlist_flow,
             "has_vhdl2008": has_vhdl2008,
             "has_xci": has_xci,
             "bd_files": bd_files,