[L] Change in coreboot[master]: screebo: initial coreboot for rex variant

[Subrata Banik (Code Review)]

Attention is currently required from: YH Lin, Tarun Tuli, Kapil Porwal, Simon Zhou.

Subrata Banik has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/74392 )

Change subject: screebo: initial coreboot for rex variant
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Patch Set 2:

(1 comment)

Patchset:

PS2: 
@Simon, do u have platform mapping document for Screebo? which we can refer to review the code?