option: Introduce CMOS_LAYOUT_FILE Kconfig symbol

Mainboards with variants may not always use the same cmos.layout file.
Turn the hardcoded path into a Kconfig symbol to allow changing it.

Tested with BUILD_TIMELESS=1: Without including the config file in the
coreboot.rom and with `USE_OPTION_TABLE` selected, building for the Asus
P8H61-M PRO produces an identical coreboot image.

Change-Id: I4cc622dcb70855c06cb8a816c34406f8421180df
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54366
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/Kconfig

@@ -682,6 +682,11 @@ config HAVE_OPTION_TABLE
 	  file containing NVRAM/CMOS bit definitions.
 	  It defaults to 'n' but can be selected in mainboard/*/Kconfig.
 
+config CMOS_LAYOUT_FILE
+	string
+	default "src/mainboard/\$(MAINBOARDDIR)/cmos.layout"
+	depends on HAVE_OPTION_TABLE
+
 config PCI_IO_CFG_EXT
 	bool
 	default n