Thanks for your wonderful instruction, Peter. Flashing U-Boot on SPI works at the first try. I got my pinebook pro yesterday and is now running fedora on emmc. So far, it's working as you described in your write-up.
There is one thing that caught me though. That is this pinebook pro with U-Boot on SPI does not boot iso images from USB or SD card. Initially I wanted to do a installation from Workstation LiveCD, but after the GRUB menu, the screen display remains dark and hanged indefinitely while the power LED lights up green. The only way I can boot fedora is to use arm-image-installer to write raw.xz files to USB or SD card or emmc, all of them boots.
I guess there may be some issues between the EFI and the U-Boot that causes the iso files does not boot?
Thanks again for your write-up and hard work.
Cheers.