Sorry, my bad. I logged in their FTP with a private user name. Anyway, I have uploaded the mentioned files to Vamrs Download (http://dl.vamrs.com) .
Please download the latest from open ai lab ftp: /ROCK960/firmware/Ubuntu/v13_20180518
You should do the following to flash the image:
FIrst, get the partition table from an existing image:
dd if=system.img of=gpt.bin bs=1M count=1
Then flash it
rkdeveloptool db rk3399_loader_v1.08.106.bin
rkdeveloptool wl 0 gpt.bin
rkdeveloptool wl 64 idbloader.img
rkdeveloptool wl 16384 uboot.img
rkdeveloptool wl 24576 trust.img
rkdeveloptool wl 32768 boot.img
rkdeveloptool wl 262144 Xubuntu-aarch64.img