Kevin Boone, May 2021
This is a collection of shell and Perl scripts that builds a minimal Raspberry Pi Zero image, which boots straight to the Minicom terminal emulator. The purpose is to use the Pi Zero as the terminal for another serial device, using a USB-to-serial adapter like the CH341. The Pi Zero is potentially a good choice as a terminal for, say, a CP/M system since it allows a modern HDMI monitor and USB keyboard to be connected. There are a number of minimal Rasberry Pi distributions that will work on the Pi Zero, but my goal here is to create an installation that boots to Minicom in about five seconds, and is completely read-only, so there's no shutdown protocol -- just power it off.
This build is for the (cheap) version of the Pi Zero that has no network or bluetooth support. These things aren't needed in a terminal emulator, and only add to the complexity.
You'll need as a minimum to modify CONFIG.sh to set the serial port device and baud rate. It may be necessary to add additional kernel modules, if those included do not work with the USB-to-serial you have. It should also be possible to configure this build to use the real built-in UART, rather than USB-to serial.
The Linux build boots so that Minicom is running on the virtual terminal /dev/tty1, while tty2 and tty3 have an ordinary Linux shell prompt. You can switch using ctrl+alt+FXX in the usual way. I've arranged it like this so that, if Minicom doesn't start, or doesn't work, it's at least possible to get a shell session to troubleshoot.