I have also figured out that if you have the v1.0 version of the firmware, you can tell EFI to boot the SD card by default (instead of the SPI image).
Use efibootmgr once you have the Debian image booted. You will first need to load the efivars module (modeprobe efivars), then run efibootmgr. A command like
# efibootmgr --bootorder 0003,0000,00002,00001
should do the trick. This tells EFI to boot option 0003 (the SD card in my system) first.