docs: def generate_signing_key(args):
      """ Generate an ECDSA signing key for signing secure boot images (post-bootloader) """
      if os.path.exists(args.keyfile):
          raise esptool.FatalError("ERROR: Key file %s already exists" % args.keyfile)
      sk = ecdsa.SigningKey.generate(curve=ecdsa.NIST256p)
      with open(args.keyfile, "wb") as f:
          f.write(sk.to_pem())
      print("ECDSA NIST256p private key in PEM format written to %s" % args.keyfile)