mainboard/google/poppy: Prepare sharing directory for variants

Clean up Kconfig file in order to support variants for poppy. Add
BOARD_GOOGLE_BASEBOARD_POPPY that can be set by various poppy variants
to use the common baseboard configs.

BUG=b:37375693

Change-Id: I399ecc8c3efb3af26e1fcf60fe2c75b24769fc0f
Signed-off-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: https://review.coreboot.org/19321
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
2 files changed