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

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

src/mainboard/intel/baskingridge/Kconfig

@@ -13,6 +13,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select MMCONF_SUPPORT
 	select HAVE_SMI_HANDLER
 	select CHROMEOS
+	select CHROMEOS_VBNV_CMOS
 	select EXTERNAL_MRC_BLOB
 	select LAPIC_MONOTONIC_TIMER
 

src/mainboard/intel/bayleybay/Kconfig

@@ -9,6 +9,7 @@ config BOARD_SPECIFIC_OPTIONS
 	select HAVE_OPTION_TABLE
 	select HAVE_ACPI_RESUME
 	select CHROMEOS
+	select CHROMEOS_VBNV_CMOS
 	select MARK_GRAPHICS_MEM_WRCOMB
 
 config MAINBOARD_DIR

src/mainboard/intel/emeraldlake2/Kconfig

@@ -13,6 +13,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select MMCONF_SUPPORT
 	select GFXUMA
 	#select CHROMEOS
+	#select CHROMEOS_VBNV_CMOS
 	select EXTERNAL_MRC_BLOB
 	select EARLY_CBMEM_INIT
 

src/mainboard/intel/pearlvalley/Kconfig

@@ -10,6 +10,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select MMCONF_SUPPORT
 	select HAVE_SMI_HANDLER
 	select CHROMEOS
+	select CHROMEOS_VBNV_CMOS
 	select EXTERNAL_MRC_BLOB
 	select CACHE_ROM
 	select MARK_GRAPHICS_MEM_WRCOMB

src/mainboard/intel/wtm2/Kconfig

@@ -10,6 +10,7 @@ config BOARD_SPECIFIC_OPTIONS # dummy
 	select MMCONF_SUPPORT
 	select HAVE_SMI_HANDLER
 	select CHROMEOS
+	select CHROMEOS_VBNV_CMOS
 	select EXTERNAL_MRC_BLOB
 	select CACHE_ROM
 	select MARK_GRAPHICS_MEM_WRCOMB