BACKPORT: 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: If8e16cb321bb53eed4013dc5ea2436a4f40eeb6b
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/49738
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

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);