[3/4] Add chapter on merging back changes from XenServer patch-queue.

[casasnovas]

Our dom0 kernel will now diverge significantly from the XenServer one, as we will be rebasing it
on top of different upstream bases, and as such, we will have some extra challenges merging back
changes from XenServer patch-queue. A new script is added to facilitate this maintenance activity.

The rationale for diverging from XenServer dom0 kernel is discussed here for reference:

https://notes.vates.tech/s/vatesglobal/p/proposal-emancipation-from-xs-linux-kernel-for-xcp-ng-8-3-and-apply-WDv7wHUn8z

This pull-request is the third in a series adding tools and documentation on maintaining
our dom0 kernel - for extra context, the final state can be seen here (the final README
especially):

https://github.com/xcp-ng/hypervisor-dev/tree/quentin-main

I've tried to split the changes into palatable/reviewable PRs, full list:

• [1/4] Add script and documentation to generate and refresh a symbol locked list. #2
• [2/4] Introduce the git-review-rebase tool as well as documentation on rebasing to latest upstream. #3
• [4/4] Introduce the kabi tool from Oracle and custom enhancements + documentation. #5

[fallen]

I think this script needs:

• a comment that explains what it does and how to use it, maybe a small example
• (optional) arg parsing, at least so that the error is not something like line 5: $1: unbound variable, with a -h to show help/usage

EDIT: the script usage is explained externally in the README.md doc so maybe the explanation could be minimal in the script itself

[casasnovas]

I think this script needs:

* a comment that explains what it does and how to use it, maybe a small example

* (optional) arg parsing, at least so that the error is not something like `line 5: $1: unbound variable`, with a -h to show help/usage

EDIT: the script usage is explained externally in the README.md doc so maybe the explanation could be minimal in the script itself

Indeed, I've added a comment banner with basic explanation (purpose and how to use) as well as a couple of pitfalls checks for the CLI but didn't go for full blown optarg parsing, let me know if that's enough for you or if you'd prefer optarg parsing.

[fallen]

I think this script needs:

* a comment that explains what it does and how to use it, maybe a small example

* (optional) arg parsing, at least so that the error is not something like `line 5: $1: unbound variable`, with a -h to show help/usage

EDIT: the script usage is explained externally in the README.md doc so maybe the explanation could be minimal in the script itself

Indeed, I've added a comment banner with basic explanation (purpose and how to use) as well as a couple of pitfalls checks for the CLI but didn't go for full blown optarg parsing, let me know if that's enough for you or if you'd prefer optarg parsing.

That's good as it is now, thanks!

[fallen]

lgtm!