amirdelta

To build the whole image, see https://github.com/150balbes/Build-Armbian but its a bit involved. You can google forum.armbian  + github.com/150balbes/Amlogic_s905-kernel which is just the kernel. Can't tell you what branch of this kernel source tree to use and can't say if all the required patches are there (as they may be applied in the
Build-Armbian repo).

Have a look at https://forum.armbian.com/topic/12162-single-armbian-image-for-rk-aml-aw/page/10/?tab=comments#comment-96403
So that will be selinux=0 at the end of the APPEND= line in /boot/uEnv.txt
It's only a work around but it would be nice to get a fix.

You do have access to the source for these builds:  https://github.com/150balbes/Build-Armbian and https://github.com/150balbes/Amlogic_s905-kernel (actually these git trees are usually a few days behind the most recent balbes150's builds as he waits for code changes to stabilize before pushing them back to these github trees)
I would recommend you start by pulling the Build-Armbian environment, and building the equivalent of what you currently have installed.  You will see that the build system creates the kernel header package as part of the build process.  Then you can experiment with improvements and contribute back to the overall community.

He is already part of the image and works on Tanix T9S