Add an "--only" flag to quickly build just one kind of artifact

Most often I find that I want to check just one kind of artifact
(e.g., mpy files) and this is a quicker syntax than excluding the
other 3 types with 3 --ignores.

circuitpython_build_tools/scripts/build_bundles.py

@@ -235,15 +235,17 @@ def _find_libraries(current_path, depth):
             subdirectories.extend(_find_libraries(path, depth - 1))
     return subdirectories
 
+all_modules = ["py", "mpy", "example", "json"]
 @click.command()
 @click.option('--filename_prefix', required=True, help="Filename prefix for the output zip files.")
 @click.option('--output_directory', default="bundles", help="Output location for the zip files.")
 @click.option('--library_location', required=True, help="Location of libraries to bundle.")
 @click.option('--library_depth', default=0, help="Depth of library folders. This is useful when multiple libraries are bundled together but are initially in separate subfolders.")
 @click.option('--package_folder_prefix', default="adafruit_", help="Prefix string used to determine package folders to bundle.")
 @click.option('--remote_name', default="origin", help="Git remote name to use during building")
-@click.option('--ignore', "-i", multiple=True, type=click.Choice(["py", "mpy", "example", "json"]), help="Bundles to ignore building")
-def build_bundles(filename_prefix, output_directory, library_location, library_depth, package_folder_prefix, remote_name, ignore):
+@click.option('--ignore', "-i", multiple=True, type=click.Choice(all_modules), help="Bundles to ignore building")
+@click.option('--only', "-o", multiple=True, type=click.Choice(all_modules), help="Bundles to build building")
+def build_bundles(filename_prefix, output_directory, library_location, library_depth, package_folder_prefix, remote_name, ignore, only):
     os.makedirs(output_directory, exist_ok=True)
 
     package_folder_prefix = package_folder_prefix.split(", ")
@@ -263,6 +265,11 @@ def build_bundles(filename_prefix, output_directory, library_location, library_d
     with open(build_tools_fn, "w") as f:
         f.write(build_tools_version)
 
+    if ignore and only:
+        raise SystemExit("Only specify one of --ignore / --only")
+    if only:
+        ignore = set(all_modules) - set(only)
+
     # Build raw source .py bundle
     if "py" not in ignore:
         zip_filename = os.path.join(output_directory,