libc: Import strtok_r from FreeBSD project

From commit: 21571b1d140ae7bb44e94c0afba2ec61456b275b
Made small changes to fit into TF-A project

Change-Id: I991f653a7ace04f9c84bcda78ad8d7114ea18e93
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/include/lib/libc/string.h b/include/lib/libc/string.h
index 91cbafb..9894483 100644
--- a/include/lib/libc/string.h
+++ b/include/lib/libc/string.h
@@ -27,5 +27,6 @@
 char *strrchr(const char *p, int ch);
 size_t strlcpy(char * dst, const char * src, size_t dsize);
 size_t strlcat(char * dst, const char * src, size_t dsize);
+char *strtok_r(char *s, const char *delim, char **last);
 
 #endif /* STRING_H */