Bering-uClibc 6.x - User Guide - Advanced Topics - Simpler setup using the PI's serial port ttyAMA0
Raspberry PI serial port
All Raspberry PIs have at least a serial comm. port available on GPIO pins 14 (tx) and 15 (rx) that can be connected to a ttl-rs232 converter, try using one with the MAX3232 chip as it works for 5 or 3.3 volts. You then only need to properly manage the jumper cables between the PI's GPIO pins and the rs232 converter. To talk to the serial port you will then have to use /dev/ttyAMA0.
An interesting and amusing setup is with the inexpensive and miniature RPI Zero W. In LEAF 7.0.1 you can find https://sourceforge.net/projects/leaf/files/Bering-uClibc/7.0.1/Bering-uClibc_PiZ.img/download image that you will copy on an SD card, using dd or Raspberry PI imager. .... to be continued ...