Reduce code size when mbedtls_x509_*_info() unused

Introduce MBEDTLS_X509_INFO to indicate the availability of the
mbedtls_x509_*_info() function and closely related APIs. When this is
not defined, also omit name and description from
mbedtls_oid_descriptor_t, and omit OID arrays, macros, and types that
are entirely unused. This saves several KB of code space.

Change-Id: I056312613379890e0d70e1d08c34171287c0aa17
diff --git a/programs/ssl/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 55c90c6..7a84b6a 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -220,21 +220,27 @@
     /* In real life, we probably want to bail out when ret != 0 */
     if( ( flags = mbedtls_ssl_get_verify_result( ssl ) ) != 0 )
     {
+#if defined(MBEDTLS_X509_INFO)
         char vrfy_buf[512];
+#endif
 
         mbedtls_printf( " failed\n" );
 
+#if defined(MBEDTLS_X509_INFO)
         mbedtls_x509_crt_verify_info( vrfy_buf, sizeof( vrfy_buf ), "  ! ", flags );
 
         mbedtls_printf( "%s\n", vrfy_buf );
+#endif
     }
     else
         mbedtls_printf( " ok\n" );
 
+#if defined(MBEDTLS_X509_INFO)
     mbedtls_printf( "  . Peer certificate information    ...\n" );
     mbedtls_x509_crt_info( (char *) buf, sizeof( buf ) - 1, "      ",
                    mbedtls_ssl_get_peer_cert( ssl ) );
     mbedtls_printf( "%s\n", buf );
+#endif
 
     return( 0 );
 }