Yeah, the Jetson doesn’t use Grub. Then again, it also doesn’t use nouveau. Presumably whatever problem you’re having is actually related to something else. The Jetson uses a module named nvgpu AFAIK.
I’m actually a little surprised nouveau ships on the Jetson kernel at all, but I guess if you plug a NVIDIA card into the PCI-Express slot on the side of the module (assuming AGX here) then it could be useful.
The easiest way out is to put the module into recovery mode (Hold in the center of the three switches, then press and release the reset switch, or power switch if it’s not on yet, keeping the center switch held for about 10 more seconds.)
Then flash the device from your host Jetpack computer as usual.