[M] Change in coreboot[main]: mb: source site-local mainboard Kconfig.name and Kconfig files

[Jonathon Hall (Code Review)]

Attention is currently required from: Alexander Couzens, David Hendricks, Evgeny Zinoviev, Felix Held, Felix Singer, Jeremy Soller, Martin L Roth, Matt DeVillier, Michał Żygowski, Philipp Hug, Piotr Król, Ron Minnich, Sean Rhodes, Tim Crawford.

Jonathon Hall has posted comments on this change by Martin L Roth. ( https://review.coreboot.org/c/coreboot/+/83119?usp=email )

Change subject: mb: source site-local mainboard Kconfig.name and Kconfig files
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Patch Set 2:

(1 comment)

Patchset:

PS2: 
Could we do this from `src/mainboard/Kconfig` instead of having to add it to every vendor's Kconfig?  I think this would reduce maintenance burden and reduce boilerplate in the mainboard vendor Kconfigs.

What if we expected site-local vendor directories to have a Kconfig/Kconfig.name comparable to src/mainboard/vendor?  Then source those from `src/mainboard/Kconfig`, e.g. `source "site-local/src/mainboard/*/Kconfig"`.

A possible downside is that you wouldn't be able to integrate the mainboard choices for a vendor from both normal and site-local directories as-is, I can see two options:
* use a different vendor for site-local (e.g. `config VENDOR_<name>_PRIVATE`)
* add a name to the mainboard choice in src/mainboard/<name>/Kconfig (e.g. `choice MAINBOARD_MODEL_<vendor>`) so you can extend the same choice from site-local

If we go with the latter we might still need changes to many mainboard directories, but it seems much less prone to drift to me.