I have just seen this issue on the 5.2-rc integration branch, to get mmc booting I needed to ensure I did try to remount any disks from fstab (it would crash here). When I did get it to boot if I pushes the mmc hard it hung (waiting on mmc) and load slowly increased with a lot of io wait. SD card does not have this issue. and usb seems fine.
Trying to find a solution and this looks interesting which I am yet to try...
Edit: the problem can occur with sd card as well after some time, will try the patch above
Edit: mmc / sd problems seem to have disappeared wtih patch above
I had an attempt and building mainline uboot, adding in the N2 support and building the ATF firmware to run uboot. I did not have uboot working (some issues not long after uboot is started, I think I missed reserving memory) but I did manage to work out how to format the amlogic firmware to start uboot. It was a bit more complex and messy than the C2 process but can be scripted easy enough I believe, we would need some compiled blobs from the hardkernel git still I think for the moment as in the C2.
I will keep trying uboot as baylibre add support for the g12a there. Would be nice to have mainline running on the SPI as many more possible features for booting images.