Update kernel build instructions for Pi 5

[timonsku]

Just noticed the kernel build docs don't have instructions for the Pi5.
Quite a bit changed also in Bookworm with the use of initramfs that also affects older gens, that could probably use updated instructions overall.

[migs35323]

heh, i tried to follow the docs in a rpi4 with bullseye, and there is no /boot/firmware folder

the makefile seems to have a install target, the docs dont reflect this info

[aallan]

heh, i tried to follow the docs in a rpi4 with bullseye, and there is no /boot/firmware folder

Documentation is written to support our latest operating system Bookworm.

[lurch]

Partially fixed by #3284

[timonsku]

Yea I think whats missing is a note to either disable initramfs loading in config.txt or how to generate a new one from the freshly compiled kernel. Would have made a PR but I'm not sure what RPi prefers here

[JinShil]

I'm pretty sure generating initramfs is what RPi prefers. If you know how to do that, please submit a PR. If it's not what RPi wants, I'm sure they'll let you know in that PR.

[JinShil]

Another partial fix: #3313

I think this just needs instructions for creating/updating initramfs.

[github-actions]

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