For my application I need, that pin P8-13(GPIO23), will get output high level as soon as possible. As I understand, I need to change both uboot and kernel. (is it correct?)
But I can not find some manual for these changes.
Yes, and sort of no. With either way I'm about to mention, you will of
course have to write, or modify an overlay to make your GPIO(s) high, or
low as needed. After that.
Loading the overlay the absolute fastest, you'll want to upgrade to one of
the latest kernels, and uboot.
You can also just load the overlay from uEnv.txt as per normal, and it'll
also load nearly as fast, but perhaps a split second slower.
The actual difference here is undocumented, but in the first case, the
second stage bootloader is what loads the overlay, where in the second
case, the initramfs( initial ram disk ) is what loads the overlay.
As far as documentation goes. You'd be better off waiting for Robert's
reply. As the bootloader method as far as I'm aware is still experimental,
and subject to change.