@Efe Çetin, PR is created. https://github.com/armbian/build/pull/6276
Bear with me, it is my first PR. So let me know whatever is wrong or needs to be changed.
Here is the patch I use. I did also add the poweroff support. Splitting it into multiple files might be better, but it is good enough for my test builds.
Hello again! It turns out I solved this problem by utilizing the Extension Hook format_partitions (https://docs.armbian.com/Developer-Guide_Extensions-Hooks/#format_partitions)
Essentially you can follow the example outlined here https://docs.armbian.com/Developer-Guide_Extensions/#example where you can write all of your fstab setup logic in its own shell script, say it's called fstab-setup.sh. You'll need to write a function in this fstab-setup.sh script where the function name is formatted as format_partition__func_name() where "func_name" can be any name you like. It's important to note that you will need to have the extension hook name, in this case its format_parition, first in the function name followed by 2 underscores.
Finally, in your configuration file, or on the ./compile.sh line, you will need to add ENABLE_EXTENSIONS=fstab-setup where fstab-setup is the name of the shell script. Something like
./compile.sh ENABLE_EXTENSIONS=fstab-setup BOARD=....more args....