Now NVME feature is enabled in UEFI. The variable could be saved in eMMC, although there's one issue on it. And this feature is merged into UEFI in github of 96boards. You can test it now.
If you only create a new variable in UEFI, it works well.
If you update the variable with multiple times (more than two), sometimes you'll fail to find the variable after reboot. In this case, we have to reflush the NVME parition in eMMC. Up to now, it's the only one workaround. I'll continue to investigate this issue.
When we check the eMMC device, we could find the variable are already flushed into eMMC. Old variables are marked deleted state (0x3d) already. And new variable is marked as added state (0x3f).
Best Regards Haojian