[stm32] Uart: Add transmission complete flag

[strongly-typed]

Just a quick expose the TC flag in the API.

TXE goes high when the shift register started pouring the bytes to the line and a new byte can be written to the data register.
The TC flag goes high after the stop bit is send. Required for manually enabling and disabling line drivers.

[salkinium]

Ohai, long time, no PR!
Do you want check if there is something similiar for AVR, just for consistency?

[strongly-typed]

Hoi, yes the first public PR for a long time. modm is in heavy use here in Switzerland!

e.g. ATMEGA32 UCSRA Bit 6 – TXC: USART Transmit Complete
I'll look into it but have no HW to test.

Shall it become part of the common API?

[salkinium]

Shall it become part of the common API?

No, I just thought it would be neat to have feature parity while we're here. It's not a priority though.

[strongly-typed]

Just checked, there is e.g. no isTransmitRegisterEmpty in current modm's UART API as there is no UART HAL for AVRs (yet). So I would not add this unless we add the HAL concept to AVR. Agree?

[salkinium]

Good point, I completely forgot about the AVR HAL…