For those who are interested in running your software on AArch64 hardware directly, without any additional firmware or operating system in between, I've developed a boot loader and tool chain to test and execute unikernels on a Pine64:
On the other hand, it has different goals and is just meant to load a blob of instructions over ethernet (faster evaluation cycle) not an entire Linux kernel with all the POSIX / UNIX stuff from SD. So you have far more control but are also on a lower level, somewhat like developing for a more powerful Arduino.