Fix HAL_STM32 + Arduino IDE SoftwareSerial conflict

[sjasonsmith]

Description

Skip building the HAL_STM32/SoftwareSerial.cpp when building outside of Platform IO.

This prevents duplicate definitions when both the Marlin and Arduino_Core_STM32 versions of SoftwareSerial.cpp are compiled.

Benefits

Allows building Rumba32 inside the Arduino IDE with TMCStepper installed.

The only downside to this is that the Arduino_Core_STM32 1.7 SoftwareSerial.cpp won't work for single-pin serial. This is probably not a major issue since prior to Arduino_Core_STM32 1.7 there was no SoftwareSerial for this HAL at all, so existing boards building in Arduino are unlikely to already depend on it.

Related Issues

#15952

[AnHardt]

The only downside to this is that the Arduino_Core_STM32 1.7 SoftwareSerial.cpp won't work for single-pin serial.

That will cause problems with the BTT SKR PRO V1.1 .
There are boars around where the resistor, connecting RX and TX pins, is not populated.
As far as i know, the TX-pins have never been used. Before the latest patches a patched TMC-library was required.

However - Arduino_Core_STM32 1.8, with a "single-pin software-serial" is expected to be published this year.

[tatusah]

https://github.com/stm32duino/Arduino_Core_STM32/releases/tag/1.8.0 Seems like 1.8 was just released

[sjasonsmith]

@AnHardt, I should have clarified that the downside only applies when building inside the Arduino IDE. The SKR Pro will continue to use our patched version when built from PlatformIO.

Now that 1.8 is released (it was not when I posted this PR), this change will be very short-lived, since we should be able to delete this file altogether after testing confirms 1.8 is ok.

[AnHardt]

Right.
And i forgot , i'm the only one using my alpha variant for the BTT PRO for Arduino. (Not for public now. Overwriting the bootloader. :-( )

[sjasonsmith]

alpha variant for the BTT PRO

Hopefully by the time that is public we will have updated the framework and can remove this limitation.

I was going to try upgrading the framework this morning, but it isn't available through PlatformIO yet. Their github repo isn't structured in a way that allows us to point to it directly. I assume it will be published through PIO soon.