arch/x86: add ENV_CACHE_AS_RAM

Add ENV_CACHE_AS_RAM to indicate to compilation units if cache-as-ram
is employed for that particular stage.

BUG=b:72728953

Change-Id: I06dfa7afe2d967229549090d5aa95455687b0bb9
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25716
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

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

@@ -20,7 +20,7 @@
 #include <stdlib.h>
 #include <rules.h>
 
-#if defined(__PRE_RAM__) && IS_ENABLED(CONFIG_CACHE_AS_RAM)
+#if ENV_CACHE_AS_RAM
 asm(".section .car.global_data,\"w\",@nobits");
 asm(".previous");
 #ifdef __clang__

src/include/rules.h

@@ -253,4 +253,12 @@
 #define __SIMPLE_DEVICE__
 #endif
 
+/* x86 specific. Indicates that the current stage is running with cache-as-ram
+ * enabled from the beginning of the stage in C code. */
+#if defined(__PRE_RAM__)
+#define ENV_CACHE_AS_RAM IS_ENABLED(CONFIG_CACHE_AS_RAM)
+#else
+#define ENV_CACHE_AS_RAM 0
+#endif
+
 #endif /* _RULES_H */