Description:
1. Update configs subdir files with high speed USART2 serial baud rate. Update Makefile with high speed USART1 baud rate
2. Implement Nextion serial message processing from USART2. This allows the Nextion display to be connected to the Nextion port on the hotspot PCB (Modem board). SerialPort.cpp and SerialPort.h updated.
Tested on Simplex ZumSpot-RPi using 3.4" Nextion display and PD0DIB's "Model 8 - 3.5inch_GLOBE-SIMPLEX_v1.2" screens, with two touch areas ('REBOOT' and 'POWEROFF').
3. Update README.md.
Test: RPi 3B+ running Pi-Star:4.0.0-RC3, with ZumSpot-Rpi running 1.4.16 (updated with these changes) and using ON7LDS Nextion driver.
Feel free to improve these changes for use with PD0DIB's Nextion display pages.
