[S] Change in coreboot[main]: build system: Exempt `make *config` from strict symbol checks

[Uwe Poeche (Code Review)]

Attention is currently required from: Martin L Roth, Patrick Georgi, uwe taz.

Uwe Poeche has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/79298?usp=email )

Change subject: build system: Exempt `make *config` from strict symbol checks
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Patch Set 1: Code-Review+1

(1 comment)

Patchset:

PS1: 
This helps to get working config files with unknown symbols/config switches to prevent error if using menuconfig or olddefconfig which I got with CB:79259.
For test I built a mainboard (siemens/mc_apl4) with BUILD_TIMELESS=1 with and without the patch and compared the rom File. The results are binary identically.
Anyway two small things I noticed:
1) Before the patch I got some Numbers of unknown config switches but after menuconfig/save or olddefconfig I found many other switches which were deleted from config file. It seems that it helps also for config switches from 3rdparty modules?
2) After build it seems a artefact in coreboot tree: a untracked file include/generated/rustc_cfg.