[S] Change in coreboot[master]: mb/google/rex: Create karis4es variant

[Jakub Czapiga (Code Review)]

Attention is currently required from: Eran Mitrani, Kapil Porwal, Sumeet R Pawnikar, YH Lin.

Jakub Czapiga has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/76761?usp=email )

Change subject: mb/google/rex: Create karis4es variant
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Patch Set 2:

(1 comment)

Commit Message:

https://review.coreboot.org/c/coreboot/+/76761/comment/9459c6ee_52197a57 :
PS2, Line 15: WIP, Not tested yet
> Need more information.  […]

Jenkins succeeded and it builds all available boards, so it should be fine.
@Eran can you try commands below just to be sure?
```
make crosgcc-i386 -j $(nproc)
util/abuild/abuild -p none -t google/rex -b karis4es -x -a
util/abuild/abuild -p none -t google/rex -b karis -x -a
```