Junkman

Thanks you guys! @Werner, @NicoD
 
Here is how I did this:
 
```
$ sudo armbian-add-overlay rk3399-oc.dts
 
$ ls -1 /boot/overlay-user/
rk3399-oc.dtbo
 
$ cat /boot/armbianEnv.txt
verbosity=1
bootlogo=false
overlay_prefix=rockchip
rootdev=UUID=6c8153dd-e3ae-4b9c-8ea2-9c34a84bbed8
rootfstype=ext4
console=serial
usbstoragequirks=0x2537:0x1066:u,0x2537:0x1068:u
user_overlays=rk3399-oc
 
$ reboot
$ sudo apt install -y armbian-config
$ sudo armbian-config # System -> CPU (Choose new CPU freq)
 
# Done
```