build/generic_platform/ConstraintManager: Add prepend parameter to add_extension.

In some cases, prepend is useful when we want to replace existing IOs.

litex/build/generic_platform.py

@@ -205,7 +205,10 @@ class ConstraintManager:
         self.platform_commands = []
         self.connector_manager = ConnectorManager(connectors)
 
-    def add_extension(self, io):
+    def add_extension(self, io, prepend=False):
+        if prepend:
+            self.available = list(io) + self.available
+        else:
             self.available.extend(io)
 
     def add_connector(self, connectors):