What version of the Raspbian/Debain I need to download to use the prep_system_for_dietpi script?
I tried to download Raspbian Stretch Lite (Version: November 2018, 2018-11-13, Kernel version: 4.14), debian-9.6.0-arm64-netinst, but after writing to the sd card, none of them started…
Yes for RPi we use the Raspbian Stretch Lite image from here: https://www.raspberrypi.org/downloads/raspbian/
For x86_64 we use the Debian mini.iso from here: https://deb.debian.org/debian/dists/stretch/main/installer-amd64/current/images/netboot/
But for Orange Pi PC 2 is an ARM board that works with non of the above images. The Debian image you tried generally is for the right architecture (arm64), but there is still a special bootloader and kernel required for AllWinner H5 boards that Debian does not ship.
There are “official” images available, e.g.: http://www.orangepi.org/downloadresources/orangepipc2/2017-05-11/orangepipc2_107bbe84513d359564d21a4f.html
But those are very bad maintained and often outdated. This is the price you need to pay afterwards, where the hardware itself is cheap 
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Better go with well maintained 3rd party images, e.g. ARMbian: https://www.armbian.com/orange-pi-pc2/
Right…need to have uboot, otherwise it just doesn’t start 