Serial3 missing in stm32duino for genericSTM32F103CB

@maxgerhardt I’ve been looking to migrate a firmware from maple to the stm32duino core, but I’ve noticed that the genericSTM32F103CB board does not have the Serial3. Is that a miss in the variant definition? The chip has 3 UARTs and under mapple core they were all available.

This was already discussed at Change Default Serial Port for STM32 Arduino - #2 by maxgerhardt.

Thanks @maxgerhardt.
The build flags fixed it.