fix(SoftwareSerial): correct TX to RX switch when using half-duplex

[dominikberse]

Summary

When working with single-pin half-duplex SoftwareSerial, I noticed that after the call to listen() receiving data is not possible until data is written. In fact setRXTX(true) is only called within send(), which in turn is only called as long as active_out is set (i.e. ongoing transmission). Since at the beginning of listen() completion of any ongoing transmissions is awaited, the SoftwareSerial will not actively listen before another transmission is completed.

This PR fixes/implements the following bugs/features

• Fixes the above behavior
• No intentional breaking changes

This occurs if the communication is not initiated by the STM32. Otherwise, one would simply call listen() and then send whatever will trigger the other side to respond.

In my case, I wanted to use a second line to indicate the direction of communication (which however can not be used as a second line for full-duplex). I must admit that this is probably a very rare case, and the fix I suggest will in fact change the current behavior of calling listen() before sending data. However it took me a few hours to debug this and I thought I should at least mention this finding in case someone else stumbles upon it.

[fpistm]

LGTM

[ABOSTM]

@dominikberse
thanks for sharing your findings.
Patch LGTM