TL;DR: All the crazy bootloader shit that affected 32 bit ARM infects 64 bit ARM too.
I pretty much followed Robert Gadsdon’s instructions here, except I had to compile mkbootimg by hand from the CyanogenMod sources as it doesn’t seem to be available in Fedora.
Luckily I have all the bits you need to attach a USB serial port:
U-boot was installed as the bootloader as above.
For the disk image, I started with the virt-builder Fedora 23 aarch64 image. I copied this image to the SD card by doing:
virt-builder --arch aarch64 fedora-23 --output /dev/mmcblkX
I didn’t expect this would boot, because the kernel is wrong, and the u-boot I flashed to the Dragonboard is in any case looking for a different file. So I cross-compiled a kernel by following the Linaro instructions here (only up to the
make Image step).
I didn’t manage to get the kernel to boot, because it just doesn’t load. Or maybe it does load but doesn’t find the serial port. Whatever.