With Radxa OS you can’t disable it, but you can set the prompt & timeout to 0, which will allow it to boot while receiving data on UART
Edit /boot/extlinux/extlinux.conf and set both prompt and timeout to 0.
If you run Radxa’s script u-boot-update it’ll set both these values back to the default 1 & 10, even if you update the defaults in /usr/share/u-boot-menu/conf.d/radxa.conf as their is a bug in the script. So remember to edit extlinux.conf if you ever run u-boot-update.
Also remember to turn off UART debug console when booting kernel.
Edit /etc/kernel/cmdline and remove “console=ttyFIQ0,1500000n8“
If you are using Radxa CLI OS, I have a script that will patch the image for you and fix these issues.