I know this is an old topic but it still seems that people are still having trouble with it. So, I'll just drop this here and walk away.
Hope it helps someone!
I used the following:
Unpack Linux Upgrade Tool
Unpack Finless 2.1 ROM
Connect MK808 to USB in Recovery/Flash mode
Make a decision. 720 or 1080. I chose 720.
The next step is not required, but will make it easier.
Grab the following files from the Finless 2.1 FinlessROM directory and drop them into the Linux_Upgrade_Tool_v1.21 directory.
RK30xxLoader(L)_V1.22.bin
parameter_original
boot720.img
kernel720.img
system.img
recovery.img
misc.img
Navigate to Linux_Upgrade_Tool_v1.21 directory
Run Upgrade_Tool, as root, if required by your system
sudo ./Upgrade_Tool
Choose the device you want to flash.
Feed this to it line by line using the files that come with the Finless 2.1 ROM.
[FONT=courier new]lf # Low level format ul RK30xxLoader(L)_V1.22.bin # Load Loader di -p parameter_original # Load parameter file di -b boot720.img parameter_original # Load boot image di -k kernel720.img parameter_original # Load kernel image di -s system.img parameter_original # Load system image di -r recovery.img parameter_original # Load revocery image di -m misc.img parameter_original # Load misc image rd # Reboot device[/FONT]

This should work just fine for almost anything else too, no promises though.