Feature request: UART inversion

[digitalentity]

Many platforms (almost all STM32, RP2, ESP32) support UART TX/RX line inversion which is useful in some cases, e.g.

• interfacing with RS232 with voltage clamping instead of a proper transceiver;
• some half-duplex protocol used in radio-controlled (RC) hobbies, e.g. FrSky/Futaba S.BUS;
• interfacing with smart-cards.

We should allow enabling UART hardware inversion on supported platforms via the machine.UARTConfig.