linux-asahi: move out of boot

[dkwo]

• I tested the changes in this PR: yes
• I built this PR locally for my native architecture, (aarch64-glibc)

The move out of /boot requires changes to mkimage and docs.

[ci skip] as this is hooks.

[classabbyamp]

I'm supportive of moving kernels out of boot, but we should come up with a unified methodology for all kernels before doing it on a device-specific kernel. it should be generic enough that it doesn't need to be shoe-horned into the m1n1 hooks.

[classabbyamp]

also, if this change even happens, it should have an install message if it requires manual user intervention

[dkwo]

The generic method would be post-{install,remove} hooks (before initramfs ones) that copy stuff into /boot. I thought about it, but for asahi (and also for uefi uki) it makes more sense to have a single hook (especially when m1n1 payload is kernel, which could then be signed): this way we only write to /boot what is needed, and as a side effect the user does not try to use an unsupported initramfs.

[yushisora]

I think mkinitcpio should also be one of the supported initramfs since asahi-scripts does provide hooks for asahi.

[classabbyamp]

yes, this change (moving kernels out of boot) cannot happen unless it's supported by all initramfs hooks.

if you want kernels out of boot, make a separate proposal for a generic solution, and leave this PR just for updating the kernel

[dkwo]

I think I found a way that works with all initramfs, please have a look. Update will be moved into a separate pr.