How to modify L4T-INITRD for Xavier?

I am trying to do a custom initramfs to do a rootfs recovery like:
https://devtalk.nvidia.com/default/topic/1032951/jetson-tx1/custom-initramfs-on-jetpack-3-2-for-tx1/?offset=11#5357695
This guy mention he created a busybox initramfs with buildroot to do his rootfs recovery.

Now while experimenting with nfs boot, I once forgot to start my nfs server and I ended up in something called L4T-INITRD with a full gnu bash v3.4.42 login with all the tools I need to do what I want. That is already included in my kernel image file. So seeing this, I thought if I could start from L4T-INITRD sources and add my rootfs recovery shell script, it would be great!

Unfortunately, I can’t find documentation or source for this L4T-INITRD.

Is L4T-INITRD source and documentation available somewhere?

Thanks

Well, that was easier than I thought.
I found that L4T/bootloader/l4t_initrd.img exist.
I followed https://www.kernel.org/doc/html/latest/admin-guide/initrd.html to unpack modify and pack again.
Then I used the flash.sh -I newinitrd.img …
The

mknod /dev/kmsg c 1 11

as seen in the other post is required.