I've downloaded the Android source and built it. I have all of the Android partition images.
So where is the utility that combines these partitions into a bootable SDCard image?
First you need to download the correct file, some are for eMMc boot and some are for the SD card.
Once you do that, i had the best luck with a tool called "balenaetcher" it will uncompress the file then flash it to the card.
The other programs i tried you need a zip program to first unzip the file before you flash it, i found this one to be the most simple and easiest but its just my opinion.
Ive had my board for about a week now and ive been trying about every version of Android so far i like the "Android TV 7.1 Community Build Image [microSD Boot] by ayufan" its way down at the bottom on the wiki.pine64.org download section.
Even though im not a fan of the leanback launcher style it runs the best on and SD card ive found