Change in coreboot[master]: mb/google/brya: Create primus variant

[Scott Chao (Code Review)]

Attention is currently required from: Furquan Shaikh, Paul Menzel, Tim Wawrzynczak, Scott Chao.
Scott Chao has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/54941 )

Change subject: mb/google/brya: Create primus variant
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Patch Set 3:

(2 comments)

File src/mainboard/google/brya/Kconfig.name:

https://review.coreboot.org/c/coreboot/+/54941/comment/077ee268_67e05055 
PS2, Line 11: 	select BOARD_GOOGLE_BASEBOARD_BRYA
> You might want to add: […]

Done

File src/mainboard/google/brya/variants/primus/memory/Makefile.inc:

https://review.coreboot.org/c/coreboot/+/54941/comment/e0351a3a_fdccfbe2 
PS2, Line 5: placeholder.spd.hex
> This doesn't really exist. […]

With blank(only SPD_SOURCES =), I can not build coreboot successfully.