linux: add 6.18 test kernel alongside 6.1

[benhillis]

Summary

Adds a parallel 6.18 (linux-6.18.y, v6.18.31) test kernel build alongside the existing 6.1 LTS kernel that ships today. The two kernels build side-by-side and are released as separate openvmm-test-linux-<version> artifacts; the shared initrd is unchanged.

Changes

• Dockerfile: new src-linux-6.18 source pin (commit 6c1de6e6), build-linux-6.18 / result-linux-6.18 stages mirroring 6.1, and a COPY --from=result-linux-6.18 / /linux-6.18/ line in the output stage.
• sysroots/linux-6.18/deps: exports LINUX_VERSION=6.18 so the shared pkg/linux/build.sh selects the 6.18 config tree.
• pkg/linux/6.18/{x86_64,aarch64}.config: seeded by copying from 6.1. CI's in-build olddefconfig is canonical; if these diverge under CONFIG_WERROR=y the resolved configs should be pulled back per the "Updating a kernel config" recipe in pkg/linux/README.md.
• .github/workflows/build.yml: extend KERNELS to "6.1 6.18" so the release job packages openvmm-test-linux-6.18.<arch>.<ver>.tar.gz (gh-release upload is glob-based, no further changes needed).
• cgmanifest.json: regenerated via pkg/Tools/gen-cgmanifest.py to register the new gregkh/linux pin.
• READMEs: documented the new linux-6.18/ output, artifact, and config layout.

Notes

• 6.1 is unchanged (already at .172 after linux: split test kernel into its own release artifact #62).
• This builds the latest stable 6.18 point release at branch time (v6.18.31). Future bumps update both the ADD SHA in the Dockerfile and cgmanifest.json (regenerated).
• Originally this PR was scoped at adding 6.6 + 6.12; reset and reduced to just 6.18 per offline discussion.

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

This PR expands the Linux “test kernel” build pipeline from a single 6.1 kernel to three LTS lines (6.1/6.6/6.12), and changes the release packaging so kernels are shipped as separate artifacts from openvmm-deps.

Changes:

• Add per-kernel-version sysroots that export LINUX_VERSION, and update pkg/linux/build.sh to select the corresponding per-version config.
• Update the Dockerfile to build three pinned kernel source stages and produce separate output directories for openvmm-deps/ and each kernel.
• Update release workflow versioning and artifact layout to publish separate openvmm-test-linux-<kver> tarballs.

Reviewed changes

Copilot reviewed 10 out of 16 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
sysroots/linux/deps	Removes the legacy linux sysroot deps entry (transition to per-version sysroots).
sysroots/linux-6.1/deps	Adds a 6.1 sysroot exporting LINUX_VERSION=6.1.
sysroots/linux-6.6/deps	Adds a 6.6 sysroot exporting LINUX_VERSION=6.6.
sysroots/linux-6.12/deps	Adds a 6.12 sysroot exporting LINUX_VERSION=6.12.
pkg/linux/build.sh	Requires LINUX_VERSION and selects the matching pkg/linux/<ver>/<arch>.config.
pkg/linux/README.md	Documents new multi-version layout and how to update/add configs.
pkg/linux/6.6/aarch64.config	Adds generated kernel configuration for Linux 6.6 arm64.
cgmanifest.json	Adds additional pinned Linux git commits for component governance.
README.md	Documents new output directory layout and new release artifact set.
Dockerfile	Adds pinned sources and build stages per kernel; output stage now separates artifacts by directory.
.github/workflows/build.yml	Updates packaging to emit multiple kernel artifacts and bumps version prefix.

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[Copilot]

Pull request overview

Copilot reviewed 10 out of 16 changed files in this pull request and generated no new comments.

[benhillis]

I'm going to split this into two PRs.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 8 changed files in this pull request and generated 2 comments.

[smalis-msft]

I'm just going to assume those configs are what we want, not gonna review those whole giant things haha