I have no luck getting the fastboot mode working (I personally believe that the rockchip doesn't support fastboot mode at all).

To flash to boot.img (and works for other partition as well), you need to put your device into fastboot mode, then just follow the tutorial.

From the last screenshot that the tool can read the partition info (address) from the device. After the address is being read, then you just need to select the partition, the image and then click 'Run'. Then it would flash the corresponding partition for you.

But first of all you need to ask yourself why you want to flash in the first place?