net: lwip: provide net_start_again()

Implement net_start_again() when NET_LWIP=y in a very similar way to
NET. This will be used in a future commit to determine if a failed
ping needs to be tried again on a different interface.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/net-common.h

@@ -471,6 +471,9 @@ static inline struct in_addr env_get_ip(char *var)
 
 int net_init(void);
 
+/* Called when a network operation fails to know if it should be re-tried */
+int net_start_again(void);
+
 /* NET compatibility */
 enum proto_t;
 int net_loop(enum proto_t protocol);