docs: add example of building the Linux kernel

[Timmmm]

Slightly expand the QEMU docs to explain how to build the flat Linux kernel image.

This might seem really obvious to you pros, but I spend a good 20 minutes googling how to do this and completely failing - there doesn't seem to be much documentation on this stuff, at least not easily googleable. Almost everything uses vmlinux. I also tried reading the Linux makefiles and things like make arch/riscv/boot/Image which you'd think would work but doesn't. I never would have guessed make Image.

[github-actions]

We have mailing list based patch review so it would be great if you can send these patchs to OpenSBI mailing list.

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