This PR extends the behavior of the kernel plugin to include features desired by the Canonical Kernel team. While these options are intended for that specific usage, they are valid options to use in other use-cases. Their inclusion has expand the complexity of the kernel plugin, so documentation has been improved to try and clarify the most salient choices which impact the decisions snapcrafters should make when crafting a kernel snap. A new spread test has been added (kernel-deb, and the old kernel-deb test has been renamed kernel-bin) to cover this case in a sufficiently comprehensive way. Comments have been added to the kernel build script to identify the primary patterns of behavior the script will execute. Test coverage has been expanded to cover the new options. The initrd plugin's behavior has been modified a bit to improve usability and stability. It should be less fragile now, especially when it comes to cleaning the environment. The overall behavior and stability of the plugins has been improved. These changes have been tested by Field and the Kernel team has discussed them (I will not speak for them, we can always ask). All spread and unit tests pass locally. Docs lint and render well :) Any behavior-specific changes (like naming the installed kernel image file `vmlinuz` instead of `kernel.img`) have been debated internally so all stakeholders are aware of these things; the impact should be minimal as most users are those same stakeholders. ~~**A caveat**: with the release of Resolute, a change was made in the location and name of the Resolute release tarball used by the initrd plugin. I have been informed that this is only a *short-term divergence* and the prior naming convention will be reused at some point. This means that there will be a future PR to fix the initrd plugin once this happens. I have no timelines for this, but I have setup daily tests to check this URL :)~~ This has been resolved. This PR precedes #6255, which updates the feature branch to `HEAD`. The line count is deceptive; the new test adds a whole kernel config file. This alone is over 4k lines. CC @jicheu @atomcult @kubiko @stewarthore --- - [x] I've followed the [contribution guidelines](https://github.com/canonical/snapcraft/blob/main/CONTRIBUTING.md). - [x] I've signed the [CLA](http://www.ubuntu.com/legal/contributors/). - [x] I've successfully run `make lint && make test`. - [x] I've added or updated any relevant documentation. - [x] In documents I changed, I [added a meta description](https://canonical-starflow.readthedocs-hosted.com/how-to/add-a-page-meta-description/) if one was missing. - [x] I've updated the relevant release notes.