[U-Boot] A64 fastboot, faster way to compile/test

I'm trying out various versions of the A64 u-boot -- Allwinner lichee, longsleep, apritzel, etc on the Pine64. I can get all of them up to the prompt, but fastboot doesn't work in an of them. There is code from Allwinner in there for implementing fastboot, but so far I've had no luck getting it to do anything. I added a bunch of debug and the USB controller does not appear to be generating interrupts.

Has anyone worked with A64 fastboot? Any tips on what might be wrong?

I want a faster was to update the SD Card without physically moving it between machines.