Reject a numeric DEFAULT_SERIALn_PROTOCOL/HAL_SERIALn_PROTOCOL/HAL_OTGn_PROTOCOL
value during hwdef processing, and convert the in-tree boards that used numbers
to the equivalent SerialProtocol_* name (value-preserving).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>