CBMEM: Tidy up CAR migration

Move the  CAR migration call to arch -specific part of CBMEM init,
it is truly a x86 specific thing.

Change-Id: I715417e54f197b8745e0670d6b900a5660178141
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/7860
Reviewed-by: Aaron Durbin <adurbin@google.com>
Tested-by: build bot (Jenkins)

src/arch/riscv/include/arch/early_variables.h

@@ -20,10 +20,6 @@
 #ifndef ARCH_EARLY_VARIABLES_H
 #define ARCH_EARLY_VARIABLES_H
 
-#if defined(CONFIG_CAR_MIGRATION) && CONFIG_CAR_MIGRATION
-	#error "This is RISCV, silly... we don't have CAR here."
-#endif
-
 #define CAR_GLOBAL
 
 #define CAR_MIGRATE(migrate_fn_)
@@ -31,6 +27,5 @@ static inline void *car_get_var_ptr(void *var) { return var; }
 #define car_get_var(var) (var)
 #define car_sync_var(var) (var)
 #define car_set_var(var, val) do { (var) = (val); } while (0)
-static inline void car_migrate_variables(void) { }
 
 #endif