vboot: Introduce kconfig variable for VBNV backing storage

This introduces a new kconfig variable to select the VBNV backing
store explicitly instead of inferring it from CPU/SoC architecture.

x86 platforms have historically relied only on CMOS to store VBNV
variables, while ARM-based platforms have traditionally relied on
the EC. Neither of those solutions are going to scale well into
the future if/when CMOS disappears and we make ARM-based systems
without an EC.

BUG=chrome-os-partner:29546
BRANCH=none
TEST=compiled for nyan_blaze and samus

Original-Signed-off-by: David Hendricks <dhendrix@chromium.org>
Original-Change-Id: I4a8dadfb6bb666baf1ed4bec98b29c145dc4a1e7
Original-Reviewed-on: https://chromium-review.googlesource.com/213877
Original-Reviewed-by: Stefan Reinauer <reinauer@google.com>
Original-Commit-Queue: Vadim Bendebury <vbendeb@chromium.org>
Original-Tested-by: Vadim Bendebury <vbendeb@chromium.org>
(cherry picked from commit d088fc71b2e2b45e826d3dedb8e536ad58b8d296)

Change-Id: Iea325a8c4d07055143e993d89b827f86b8312330
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/8777
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/vendorcode/google/chromeos/Kconfig

@@ -50,6 +50,18 @@ config VBNV_SIZE
 	  CMOS storage size for VbNv data. This value must match cmos.layout
 	  in the mainboard directory.
 
+config CHROMEOS_VBNV_CMOS
+	bool "Vboot non-volatile storage in CMOS."
+	default n
+	help
+	  VBNV is stored in CMOS
+
+config CHROMEOS_VBNV_EC
+	bool "Vboot non-volatile storage in EC."
+	default n
+	help
+	  VBNV is stored in EC
+
 config CHROMEOS_RAMOOPS
 	bool "Reserve space for Chrome OS ramoops"
 	default y