string: Add STRINGIFY macro

STRINGIFY makes a string from a token. It is generally useful.
Even though STRINGIFY is not defined to be in the C library it's
placed in string.h because it does make a string.

Change-Id: I368e14792a90d1fdce2a3d4d7a48b5d400623160
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/3144
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/include/string.h

@@ -4,6 +4,12 @@
 #include <stddef.h>
 #include <stdlib.h>
 
+/* Stringify a token */
+#ifndef STRINGIFY
+#define _STRINGIFY(x)	#x
+#define STRINGIFY(x)	_STRINGIFY(x)
+#endif
+
 void *memcpy(void *dest, const void *src, size_t n);
 void *memmove(void *dest, const void *src, size_t n);
 void *memset(void *s, int c, size_t n);