In the firefly wiki they write:
Firefly-RK3399 supports booting from the following storage devices:
- eMMC
If this is true, you can’t boot from SD. This board also has no jumper to change the boot order:
PDF
It seem’s like the SD card slot is there for updating the firmware / write stuff to the eMMC.
Edit: In the wiki they have instructions how to update the firmware to use Debian and Ubuntu. Maybe you can install Debian and then run the Dietpi update script.