kernel: move dts to overlay and change extension to dtso

[zhouruixi]

The device tree overlay source code will be uniformly placed in dts/rockchip/overlay/*.dtso, and the generated files will be located in /boot/rockchip/overlay/*.dtbo.

This helps us differentiate and manage DTB and DTBO files.

[zhouruixi]

@henkwiedig Need help. I initially thought that using the same approach as opi3b would achieve the desired results, but it consistently produces errors, and I don't know how to fix it.

And do you think it's necessary for us to use the directory structure I mentioned?

[henkwiedig]

Strange, it "works" for me when i rename the overlay to *dts and remove BR2_LINUX_KERNEL_INTREE_DTSO_NAMES.
however this builds dtb files not dtbo files (they should work just the same i hope).

When i interpet the buildroot code correctly all files in BR2_LINUX_KERNEL_CUSTOM_DTS_DIR will be copied to arch/arm64/boot/dts and are added to LINUX_DTS_LIST or LINUX_DTSO_LIST depending on their suffix.
However dtso files fails to build where dts files work.

Why the opi3b work ? no clue currently. The kernel has a Makefile in arch/arm64/boot/dts/rockchip/Makefile which includes the overlay dir and the overlay dir also has a makfile. Also the opi3b build overlays in this directory by default. This is missing on the radxa kernel.
Yet this does not explain it. Maybe opi3b has a default config or radxa lacks a default config/makefile in thier kernel src which handles this.