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

[Adam Ford]

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