setting outputs

Hello everybody,

how to set inputs/outputs or ADC in kernel (4.x) without installing cape manager.

I’m assuming that the right way is editing *-overlay.dts in (bb-kernel/KERNEL/arch/arm/boot/dts).
I see that u-boot also contains (/u-boot/arch/arm/dts) *-boneblack.dts files for what is those files when we have dts files in (bb-kernel/KERNEL/arch/arm/boot/dts)?

Is there anywhere manual about this or even better a book?