Did you use the SDK Manager to do the upgrade? Or did you use the Ubuntu mechanism? The Ubuntu mechanism will fail since it has no understanding of the custom hardware. SDK Manager 4.2 understands the hardware and is the method for flashing Ubuntu 18.04 (a.k.a., L4T R32.1 when NVIDIA-specific support is added).
Note that embedded systems do not have a BIOS or UEFI. This means all of the boot support a PC motherboard has is instead custom software on an embedded system.
See:
Installer: [url]https://developer.nvidia.com/embedded/dlc/nv-sdk-manager[/url]
Instructions 1 (Download/Run): [url]http://docs.nvidia.com/sdk-manager/download-run-sdkm/index.html[/url]
Instructions 2 (Install Software to Jetson): [url]http://docs.nvidia.com/sdk-manager/install-with-sdkm-jetson/index.html[/url]