Handle when the url key is not present in board.json (#66)

src/mpbuild/board_database.py

@@ -110,7 +110,7 @@ def factory(port: Port, filename_json: Path) -> Board:
         board = Board(
             name=filename_json.parent.name,
             variants=[],
-            url=board_json["url"],
+            url=board_json.get("url", "http://micropython.org"),
             mcu=board_json["mcu"],
             product=board_json["product"],
             vendor=board_json["vendor"],