Vivado Tool: Allow for "none" option for synth (#505)

Summary

    In the case in which the synth and pnr are set to none, use the post_build as the target for the vivado runner

Reasoning

The vivado tool currently fails to run any of the post_build scripts if synth is set to none. This allows custom user injection of post_build scripts which can run custom tcls, etc. to allow a broader use of vivado features while still leveraging project mode through fusesoc's project make target.

Author: ArkoshEternal

edalize/vivado.py

@@ -111,8 +111,14 @@ def build_main(self):
         if "pnr" in self.tool_options:
             if self.tool_options["pnr"] == "vivado":
                 pass
-            elif self.tool_options["pnr"] == "none":
+            elif (
+                self.tool_options["pnr"] == "none"
+                and self.tool_options["synth"] != "none"
+            ):
                 args.append("synth")
+            else:
+                args.append("post_build")
+
         self._run_tool("make", args)
 
     def run_main(self):