I went through this on the same board back about a year ago, had to wait for 4.20 firmware to be released to try and then couldn’t find the internal eMMC. Since the eMMC is connected through a PCIe4 port, you can just replace it with any nvme 2242 SSD and it will work fine.
Mrchromebox told me when I was first having this issue that it was due to an eMMC driver not available at the time and that future updates to alder lake could improve that.
no, the device appears to the firmware as a NVMe device, since it’s using a NVMe to eMMC bridge. It’s possible that bridge device needs some tuning on the coreboot side.
I just tried the Ganondwarf solution, changed to another NVMe SSD and it was detected in BIOS. With that, I was able to install every OS I tried (Arch, Debian, Ubuntu, and also Windows), which worked flawlessly.
if someone in the US has one of these problematic NVMe->eMMC drives and wants to lend it to me for debugging, just sent me a PM