The SOM-RK3399 displayed FriendlyElec(that is the rogo before OS boot like Android) then nothing happen.
I guess the device couldn't boot. but I didn't find any data to solve the problem for friendlyDesktop OS.
So the way is Bold. just Flash FriendlyDesktop ROM again with AndroidTool_Release_v2.71 or more. that's the way I did.
After that the OS was boot without any problem.
Probably, it started when I changed all the permissions under /etc using chmod -R 777 /etc.
After that, a number of problems occurred, and the permissions under/ etc were changed many times by chmod. (I forgot 400 or 555 or 444 or other).
That is very ridiculous.
Probably, in linux, it is highly possible that the startup script is stored under /etc.
that is the reason the device couldn't boot I think.