[U-Boot] burning eMMC from sdcard.img in U-Boot

I have a board with eMMC storage. I've been trying to find an elegant solution to burning the sdcard image to eMMC. I looked at using the USB mass storage gadget, but it's a bit slow. I was hoping to avoid having to boot all the way into Linux to do it, but the sdcard.img file is larger than the memory I have available.

Does anyone have any suggestions on how to burn the contents of a file to eMMC (or mmc/sd) from within U-Boot when the source file size may vary and it's larger than the available memory, so it needs to be done in blocks?

thanks

adam