Arduino esp32 board version 2.0.15 issue

[kaiaai]

FYI, don't use Arduino esp32 board (toolchain) version 2.0.15 because HardwareSerial is broken in that version.

In other words, none of these will work when compiled with esp32 toolchain version 2.0.15:

• https://github.com/kaiaai/LDS Arduino library with its example(s)
• https://github.com/kaiaai/firmware under kaiaai-esp32 Arduino/Micro-ROS firmware

I see that HardwareSerial was fixed in version 2.0.16.