google/gru: Incorporate feedback to #14279

To avoid diverging too much on an actively developed code base, keep
the changes to a separate commit that can be downstreamed more easily:

- removed unused includes
- gave kevin board a "Kevin" part number
- marked RW_LEGACY as CBFS region (to follow up upstream changes)
- moved romstage entry point to SoC code (instead of encouraging
  per-board copy pasta)

Change-Id: Ief0c8db3c4af96fe2be2e2397d8874ad06fb6f1f
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-on: https://review.coreboot.org/14362
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
diff --git a/src/mainboard/google/gru/Kconfig b/src/mainboard/google/gru/Kconfig
index c225d10..6f234ad 100644
--- a/src/mainboard/google/gru/Kconfig
+++ b/src/mainboard/google/gru/Kconfig
@@ -41,6 +41,7 @@
 
 config MAINBOARD_PART_NUMBER
 	string
+	default "Kevin" if BOARD_GOOGLE_KEVIN
 	default "Gru"
 
 config MAINBOARD_VENDOR