Add SERIAL_PORT -1, set on SBASE, STM32F103RET6

Marlin/Configuration.h

Marlin/src/config/default/Configuration.h

Marlin/src/config/examples/AlephObjects/TAZ4/Configuration.h

Marlin/src/config/examples/AliExpress/CL-260/Configuration.h

Marlin/src/config/examples/Anet/A6/Configuration.h

Marlin/src/config/examples/Anet/A8/Configuration.h

Marlin/src/config/examples/Azteeg/X5GT/Configuration.h

Marlin/src/config/examples/BQ/Hephestos/Configuration.h

Marlin/src/config/examples/BQ/Hephestos_2/Configuration.h

Marlin/src/config/examples/BQ/WITBOX/Configuration.h

Marlin/src/config/examples/Cartesio/Configuration.h

Marlin/src/config/examples/Creality/CR-10/Configuration.h

Marlin/src/config/examples/Felix/Configuration.h

Marlin/src/config/examples/Felix/DUAL/Configuration.h

...in/src/config/examples/FolgerTech/i3-2020/Configuration.h

Marlin/src/config/examples/Geeetech/GT2560/Configuration.h

.../config/examples/Geeetech/I3_Pro_X-GT2560/Configuration.h

Marlin/src/config/examples/Infitary/i3-M508/Configuration.h

Marlin/src/config/examples/Malyan/M150/Configuration.h

...in/src/config/examples/Micromake/C1/basic/Configuration.h