Allow compile-time configuration of I/O function pointers

Introduce the compile-time options
- MBEDTLS_SSL_CONF_RECV
- MBEDTLS_SSL_CONF_SEND
- MBEDTLS_SSL_CONF_RECV_TIMEOUT
which can be used to configure the callbacks for the underlying
transport at compile-time.

Code-size impact:

|  | GCC 8.2.1 | ARMC5 5.06 | ARMC6 6.12 |
| --- | --- | --- | --- |
| `libmbedtls.a` before | 23471 | 24077 | 27045 |
| `libmbedtls.a` before | 23379 | 23981 | 26941 |
| gain in Bytes | 92 | 96 | 104 |

programs/ssl/ssl_client2.c

@@ -466,6 +466,10 @@ static void my_debug( void *ctx, int level,
     fflush(  (FILE *) ctx  );
 }
 
+
+#if !defined(MBEDTLS_SSL_CONF_RECV) && \
+    !defined(MBEDTLS_SSL_CONF_SEND) && \
+    !defined(MBEDTLS_SSL_CONF_RECV_TIMEOUT)
 /*
  * Test recv/send functions that make sure each try returns
  * WANT_READ/WANT_WRITE at least once before sucesseding
@@ -503,6 +507,9 @@ static int my_send( void *ctx, const unsigned char *buf, size_t len )
         first_try = 1; /* Next call will be a new operation */
     return( ret );
 }
+#endif /* MBEDTLS_SSL_CONF_RECV &&
+          MBEDTLS_SSL_CONF_SEND &&
+          MBEDTLS_SSL_CONF_RECV_TIMEOUT */
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 static unsigned char peer_crt_info[1024];
@@ -1876,12 +1883,18 @@ int main( int argc, char *argv[] )
     }
 #endif
 
+#if !defined(MBEDTLS_SSL_CONF_RECV) && \
+    !defined(MBEDTLS_SSL_CONF_SEND) && \
+    !defined(MBEDTLS_SSL_CONF_RECV_TIMEOUT)
     if( opt.nbio == 2 )
         mbedtls_ssl_set_bio( &ssl, &server_fd, my_send, my_recv, NULL );
     else
         mbedtls_ssl_set_bio( &ssl, &server_fd,
                              mbedtls_net_send, mbedtls_net_recv,
                              opt.nbio == 0 ? mbedtls_net_recv_timeout : NULL );
+#else
+     mbedtls_ssl_set_bio_ctx( &ssl, &server_fd );
+#endif
 
 #if defined(MBEDTLS_SSL_DTLS_CONNECTION_ID)
     if( opt.transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )