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/arch/x86/Kconfig

@@ -200,6 +200,7 @@ config VERSTAGE_ADDR
 # from a program loaded in RAM and subsequently loading ramstage.
 config POSTCAR_STAGE
 	def_bool n
+	select NO_CAR_GLOBAL_MIGRATION
 
 config VERSTAGE_DEBUG_SPINLOOP
 	bool

src/arch/x86/include/arch/early_variables.h

@@ -34,7 +34,7 @@ asm(".previous");
  * accessed unconditionally because cbmem is never initialized until romstage
  * when dram comes up.
  */
-#if ENV_VERSTAGE || ENV_BOOTBLOCK
+#if ENV_VERSTAGE || ENV_BOOTBLOCK || IS_ENABLED(CONFIG_NO_CAR_GLOBAL_MIGRATION)
 static inline void *car_get_var_ptr(void *var)
 {
 	return var;