BarTender

Just had the same problem as well, was running Buster without an issue on my C2 and needed to move to Focal for a few annoying reasons using the non desktop download https://redirect.armbian.com/region/AS/odroidc2/Focal_current and can see I also have the same issue with uboot failing to find the network adapter. I can pull the version out of the running Bionic image and then dd it onto the eMMC.
I eventually got my host working by taking the uboot from Bionic:
wget https://armbian.systemonachip.net/archive/odroidc2/archive/Armbian_20.02.8_Odroidc2_bionic_current_5.4.28_desktop.7z 7z e Armbian_20.02.8_Odroidc2_bionic_current_5.4.28_desktop.7z dd if=Armbian_20.02.8_Odroidc2_bionic_current_5.4.28_desktop.img of=bionicuboot.bin bs=512 seek=1 skip=1 count=4096 then put onto the emmc: dd if=bionicuboot.bin of=/dev/sdb bs=512 seek=1 skip=1  
Further edit.. I am going to search for my serial console tonight when I get home to help debug the root cause. I am also running the same C2 board revision as @guidol as per this post: 
 

Awesome Igor, much appreciated. That has saved a whole lot of time rebuilding u-boot and I completely missed that in the documentation .... and I can now see the SATA SSD:
smartctl -a /dev/sda === START OF INFORMATION SECTION === Device Model: SAMSUNG MZMPA128HMFU-000H1 Fantastic, now I have plenty of storage for the Router / MITM / Docker host.