You may find this of use:
[url]http://http.download.nvidia.com/tegra-public-appnotes/t210-nvtboot-flow.html[/url]
Once you get to U-Boot itself it reads “/boot/extlinux/extlinux.conf”, which is where it gets configuration which runs on top of the environment variables within U-Boot itself. Serial console once again helps here since you can see time taken to reach U-Boot and time spent actually in U-Boot. I’m not sure of order of device probes, but serial console might provide clues as to whether this is in nvtboot or U-Boot. I imagine it is possible to edit the U-Boot source or configuration and put a custom U-Boot in which eliminates some of the steps, but you might be surprised at how much can be eliminated by editing environment variables within U-Boot via serial console (many of these are macros which execute searches for available devices, so on…but if you already know which device you want, then you can edit out probing for other devices). Serial console is your friend! :)