Change #ifdef and #if defined CONFIG_ bools to #if IS_ENABLED()

Kconfigs symbols of type bool are always defined, and can be tested with
the IS_ENABLED() macro.

symbol type except string.

Change-Id: Ic4ba79f519ee2a53d39c10859bbfa9c32015b19d
Signed-off-by: Martin Roth <gaumless@gmail.com>
Reviewed-on: http://review.coreboot.org/10885
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/cpu/x86/mtrr/earlymtrr.c

@@ -19,7 +19,7 @@ void set_var_mtrr(
 	wrmsr(MTRRphysMask_MSR(reg), maskm);
 }
 
-#if !defined(CONFIG_CACHE_AS_RAM) || !CONFIG_CACHE_AS_RAM
+#if !IS_ENABLED(CONFIG_CACHE_AS_RAM)
 static void cache_ramstage(void)
 {
 	/* Enable caching for lower 1MB and ram stage using variable mtrr */