vendorcode/google: add common smbios mainboard version support

Provide an option to deliver the mainboard smbios version in the
form of 'rev%d' based on the board_id() value.

BUG=chromium:663243

Change-Id: If0a34935f570612da6e0c950fd7e8f0d92b6984f
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/17290
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins)

src/vendorcode/google/Kconfig

@@ -13,3 +13,11 @@
 ##
 
 source src/vendorcode/google/chromeos/Kconfig
+
+config GOOGLE_SMBIOS_MAINBOARD_VERSION
+	bool
+	default n
+	depends on GENERATE_SMBIOS_TABLES
+	help
+	  Provide a common implementation for mainboard version,
+	  which returns a formatted 'rev%d' board_id() string.