Wiki¶

Using ST-tools to flash images to SPI would be really nice. The Bootloader protocol is not that complicated.
https://www.st.com/en/development-tools/stm32cubeprog.html
https://www.st.com/resource/en/application_note/an3155-usart-protocol-used-in-the-stm32-bootloader-stmicroelectronics.pdf

The code should run on PC in order to test it. The serial interface can be a pseudo device:

socat -d -d pty,rawer,link=host,echo=0 pty,rawer,link=target,echo=0
${HOME}/STMicroelectronics/STM32Cube/STM32CubeProgrammer/bin/STM32_Programmer_CLI -c port=$PWD/host -e all -d theme_noto/media.bin 0x0
tio -t -l tio.log $PWD/target