The EXTERNAL_KERNEL_TAG variable is no longer needed.
For LAVA and bare-metal, we can override the KERNEL_TAG variable to fetch
both the kernel image and modules from a different tag than the default
mainline gfx-ci/linux kernel.
For LAVA, this also avoids the issue where jobs using EXTERNAL_KERNEL_TAG
would still have mainline kernel modules downloaded by the LAVA overlay.
Signed-off-by: Valentine Burley <valentine.burley@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34873>
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