Allow injection of custom kernel into pipeline

This merge request aims to rebase and adjust certain parts that were previously merged in link to previous !24079 (merged).

Pros:

1. It makes testing and bisecting faulty kernels in the CI environment much easier.
2. In the future, it enables the possibility of injecting non-Mesa3D-CI kernels, such as KernelCI or for someone who wants to test specific patches.
3. It helps to remove hacks in the bare-metal farm for injecting kernels, resulting in cleaner code and coherent behavior across multiple farm.

The above improvements are the main goals of this merge request.

Missing bits (to be done later):

• Raspberry Pi 3 and 4 support
• Valve support (very different kernel config + bundled firmwares in initrd)
• before merging DO NOT MERGE commits will be dropped.