test: x509: add more tests for checking certificate serial

- added 2 new certificates: 1 for testing a serial which is full lenght
  and another one for a serial which starts with 0x80

- added also proper Makefile and openssl configuration file to generate
  these 2 new certificates

Signed-off-by: Valerio Setti <vsetti@baylibre.com>

library/x509write_crt.c

@@ -545,7 +545,7 @@ int mbedtls_x509write_crt_der(mbedtls_x509write_cert *ctx,
         if (c - buf < 1) {
             return MBEDTLS_ERR_X509_BUFFER_TOO_SMALL;
         }
-        *(c--) = 0x0;
+        *(--c) = 0x0;
         len++;
         MBEDTLS_ASN1_CHK_ADD(len, mbedtls_asn1_write_len(&c, buf,
                                                          ctx->serial_len + 1));