Escape special characters RFC 4514

This escapes special characters according to RFC 4514 in
mbedtls_x509_dn_gets and de-escapes in mbedtls_x509_string_to_names.
This commit does not handle hexpairs.

Signed-off-by: Agathiyan Bragadeesh <agathiyan.bragadeesh2@arm.com>
diff --git a/library/x509_create.c b/library/x509_create.c
index bd772d3..170a6bc 100644
--- a/library/x509_create.c
+++ b/library/x509_create.c
@@ -153,8 +153,8 @@
         if (!in_tag && *c == '\\' && c != end) {
             c++;
 
-            /* Check for valid escaped characters */
-            if (c == end || *c != ',') {
+            /* Check for valid escaped characters in RFC 4514 in Section 3*/
+            if (c == end || !strchr(" ,=+<>#;\"\\+", *c)) {
                 ret = MBEDTLS_ERR_X509_INVALID_NAME;
                 goto exit;
             }