There are afewreferences to touching the file /opt/nvidia/l4t-packages/.nv-l4t-disable-boot-fw-update-in-preinstall when using a custom rootfs, but I can’t tell if it is actually documented anywhere.
The last link above says,
If I flash with sudo ./flash.sh -R my-custom-rootfs/ is that considered a “non-standard partition layout”?
This script creates the file before installing packages to the chroot, and then removes it. I think that makes sense if a package (nvidia-l4t-firmware?) performs an unintended preinstall action.