The Catena_Serial abstaction doesn't work with STM32 UsbSerial ports

The abstraction (necessarily) exports `cSerial::begin(unsigned long baudrate, uint16_t config)`.  But STM32 `UsbSerial` doesn't have a two-parameter begin(). This results in a compile error when trying to create a `cSerial<UsbSerial>`.

Need to use template metaprogramming to detect the lack of a method and use `UsbSerial::begin(baudrate)` instead.