stm32: Add optional lwip loopback support.

MICROPY_PY_LWIP_LOOPBACK must be set at the make level to enable this.

Signed-off-by: Peter Züger <zueger.peter@icloud.com>

ports/stm32/lwip_inc/lwipopts.h

@@ -24,6 +24,9 @@
 #define LWIP_NETIF_HOSTNAME             1
 #define LWIP_NETIF_EXT_STATUS_CALLBACK  1
 
+#define LWIP_LOOPIF_MULTICAST           1
+#define LWIP_LOOPBACK_MAX_PBUFS         8
+
 #define LWIP_IPV6                       0
 #define LWIP_DHCP                       1
 #define LWIP_DHCP_CHECK_LINK_UP         1

ports/stm32/mpnetworkport.c

@@ -72,6 +72,10 @@ STATIC void pyb_lwip_poll(void) {
 
     // Run the lwIP internal updates
     sys_check_timeouts();
+
+    #if LWIP_NETIF_LOOPBACK
+    netif_poll_all();
+    #endif
 }
 
 void mod_network_lwip_poll_wrapper(uint32_t ticks_ms) {