arch/x86: remove CAR global migration when postcar stage is used

When a platform is using postcar stage it's by definition not
tearing down cache-as-ram from within romstage prior to loading
ramstage. Because of this property there's no need to migrate
CAR_GLOBAL variables to cbmem.

Change-Id: I7c683e1937c3397cbbba15f0f5d4be9e624ac27f
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19215
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>

src/cpu/Kconfig

@@ -10,6 +10,15 @@ config CACHE_AS_RAM
 	bool
 	default !ROMCC
 
+config NO_CAR_GLOBAL_MIGRATION
+	bool
+	default n
+	depends on CACHE_AS_RAM
+	help
+	  This option is selected if there is no need to migrate CAR globals.
+	  All stages which use CAR globals can directly access the variables
+	  from their linked addresses.
+
 config DCACHE_RAM_BASE
 	hex
 

src/cpu/x86/Makefile.inc

@@ -1,6 +1,8 @@
 ifeq ($(CONFIG_ARCH_ROMSTAGE_X86_32)$(CONFIG_ARCH_ROMSTAGE_X86_64),y)
+ifneq ($(CONFIG_NO_CAR_GLOBAL_MIGRATION),y)
 romstage-$(CONFIG_CACHE_AS_RAM) += car.c
 endif
+endif
 
 subdirs-$(CONFIG_PARALLEL_MP) += name
 ramstage-$(CONFIG_PARALLEL_MP) += mp_init.c