Firmware and Kernel package renames

[lurch]

raspberrypi-bootloader has been replaced with raspi-firmware.
raspberrypi-linux has been split into separate linux-image-rpi-v6, linux-image-rpi-v7, linux-image-rpi-v7l, linux-image-rpi-v8 packages.
libraspberrypi0 libraspberrypi-{bin,dev,doc} remain as-is.

initramfs now works out of the box.
auto_initramfs=1 (which is a new option, so that also needs documenting) is added to config.txt to make the firmware automatically load the right initramfs file for the kernel.

This is very closely related to #3089
Again, this is all only for Bookworm; Bullseye retains the existing package names.

[lurch]

Fixed on bookworm branch.