Add config_enabled() from Linux

This change is taken from Linux. It allows to check for Kconfig
definitions in the preprocessor and source code using the same
idiom.

Long term plan is to remove our Kconfig hack to #define values to 0,
and this helps.

This includes a tiny modification to the macros to fix romcc support.

Change-Id: I0fddbea8c8ca215cf226acf39cb329b0ba0445a5
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/1005
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/kconfig.h

@@ -0,0 +1,21 @@
+#ifndef __KCONFIG_H__
+#define __KCONFIG_H__
+
+#include <config.h>
+
+/*
+ * Getting something that works in C and CPP for an arg that may or may
+ * not be defined is tricky.  Here, if we have "#define CONFIG_BOOGER 1"
+ * we match on the placeholder define, insert the "0," for arg1 and generate
+ * the triplet (0, 1, 0).  Then the last step cherry picks the 2nd arg (a one).
+ * When CONFIG_BOOGER is not defined, we generate a (... 1, 0) pair, and when
+ * the last step cherry picks the 2nd arg, we get a zero.
+ */
+#define __ARG_PLACEHOLDER_1 0,
+#define config_enabled(cfg) _config_enabled(cfg)
+#define _config_enabled(value) __config_enabled(__ARG_PLACEHOLDER_##value)
+#define __config_enabled(arg1_or_junk) ___config_enabled(arg1_or_junk 1, 0, 0)
+#define ___config_enabled(__ignored, val, ...) val
+
+#define IS_ENABLED(option) config_enabled(option)
+#endif