When building a custom rootfs for NVIDIA Jetson devices, there are a few references online to touching the file /opt/nvidia/l4t-packages/.nv-l4t-disable-boot-fw-update-in-preinstall, but this file's purpose is not documented anywhere.
I looked through the nvidia-l4t-* Debian packages included with the Linux for Tegra r32.7.2 board support package. Some of these packages include pre- and post-install scripts that reference the file:
nvidia-l4t-init creates files like /etc/fstab if the file does not exist.
nvidia-l4t-initrd skips flashing the new initrd if the file does exist.
nvidia-l4t-core does compatibility checks against /proc if the file does not exist.
nvidia-l4t-xusb skips flashing if the file does exist.
nvidia-l4t-apt-source inserts the correct SOC family name into /etc/apt/sources.list.d/nvidia-l4t-apt-source.list when the file does not exist.
In summary, the file mostly indicates whether you are installing a package on a live system or building a new rootfs where the host system is not the target Jetson device.