So yeah there's some tricks required to get the eMMC version up and running, as you might have noticed the SD card doesn't load.
That's because the eMMC on-board chip is using the same circuit as the SD card and thus overrides it.
So your only option to get the U-Boot booting is through UART, Solid-Run has provided some information regarding this on their wiki (read page 2 of this thread).
I found that I had to compile the latest U-Boot myself, the compilation procedure is detailed in their wiki as well. Make sure you're using a GCC 4 toolchain as their U-Boot fork does
not contain the GCC 5 patches yet.
Once you get it booted you will need to flash the U-Boot firmware to the first sector of your eMMC (also detail in the wiki, although slightly incorrect).
Now once you've got U-boot on your eMMC you can boot it as usual without UART. From here you need to get an image written into your eMMC, now here's where it gets really tricky.
I tried many approaches to write an image from U-Boot but I was unable to do anything from here due to an error "sdhci_transfer_data: Error detected in status(0x408000)!".
This guy has documented the same approach I had to take in order to get it loaded, basically I booted OpenWRT from an initram image through TFTP, partitioned the disks from there and wrote the image to the partition.
So all in all, the setup process for this is extremely dull and boring and I'm really glad I only had to do this once!