Codesign: Add codesign for macOS worker

Works similarly to Windows configuration where buildbot worker and
codesign machines are communicating with each other using network
drive.

build_files/buildbot/codesign_server_windows.py

@@ -30,15 +30,25 @@ import shutil
 from pathlib import Path
 from typing import List
 
+import codesign.util as util
+
 from codesign.windows_code_signer import WindowsCodeSigner
 import codesign.config_server
 
 if __name__ == "__main__":
+    logging.config.dictConfig(codesign.config_server.LOGGING)
+
+    logger = logging.getLogger(__name__)
+    logger_server = logger.getChild('server')
+
     # TODO(sergey): Consider moving such sanity checks into
     # CodeSigner.check_environment_or_die().
     if not shutil.which('signtool.exe'):
-        raise SystemExit("signtool.exe is not found in %PATH%")
+        if util.get_current_platform() == util.Platform.WINDOWS:
+            raise SystemExit("signtool.exe is not found in %PATH%")
+        logger_server.info(
+            'signtool.exe not found, '
+            'but will not be used on this foreign platform')
 
-    logging.config.dictConfig(codesign.config_server.LOGGING)
     code_signer = WindowsCodeSigner(codesign.config_server)
     code_signer.run_signing_server()