CI: move the kernel build part into a proper shell script
This way it can be re-used by other projects. This is a 1:1 move, the only change are the checks for KERNEL_URL and KERNEL_VERSION at the top.
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This way it can be re-used by other projects. This is a 1:1 move, the only change are the checks for KERNEL_URL and KERNEL_VERSION at the top.