Fixed issue with reconnecting the ethernet interface

Previously the EthernetInterface class was unable to be connected
after being brought down. The core issue is that lwip was not designed
to be completely brought down due once initialized.

To work around this, the ethernet interface only initializes once and
leaves itself up after a disconnect call. The DHCP lease is still
released/acquired on disconnect/connect.

Author: geky

features/net/FEATURE_IPV4/lwip-interface/lwip_stack.c

@@ -144,34 +144,33 @@ const char *lwip_get_ip_address(void)
 int lwip_bringup(void)
 {
     // Check if we've already connected
-    if (lwip_get_mac_address()) {
-        return 0;
-    }
-
-    // Set up network
-    lwip_set_mac_address();
+    if (!lwip_get_mac_address()) {
+        // Set up network
+        lwip_set_mac_address();
 
-    sys_sem_new(&lwip_tcpip_inited, 0);
-    sys_sem_new(&lwip_netif_linked, 0);
-    sys_sem_new(&lwip_netif_up, 0);
+        sys_sem_new(&lwip_tcpip_inited, 0);
+        sys_sem_new(&lwip_netif_linked, 0);
+        sys_sem_new(&lwip_netif_up, 0);
 
-    tcpip_init(lwip_tcpip_init_irq, NULL);
-    sys_arch_sem_wait(&lwip_tcpip_inited, 0);
+        tcpip_init(lwip_tcpip_init_irq, NULL);
+        sys_arch_sem_wait(&lwip_tcpip_inited, 0);
 
-    memset(&lwip_netif, 0, sizeof lwip_netif);
-    netif_add(&lwip_netif, 0, 0, 0, NULL, eth_arch_enetif_init, tcpip_input);
-    netif_set_default(&lwip_netif);
+        memset(&lwip_netif, 0, sizeof lwip_netif);
+        netif_add(&lwip_netif, 0, 0, 0, NULL, eth_arch_enetif_init, tcpip_input);
+        netif_set_default(&lwip_netif);
 
-    netif_set_link_callback  (&lwip_netif, lwip_netif_link_irq);
-    netif_set_status_callback(&lwip_netif, lwip_netif_status_irq);
+        netif_set_link_callback  (&lwip_netif, lwip_netif_link_irq);
+        netif_set_status_callback(&lwip_netif, lwip_netif_status_irq);
 
-    // Connect to network
-    eth_arch_enable_interrupts();
-    dhcp_start(&lwip_netif);
+        eth_arch_enable_interrupts();
+    }
 
     // Zero out socket set
     lwip_arena_init();
 
+    // Connect to the network
+    dhcp_start(&lwip_netif);
+
     // Wait for an IP Address
     u32_t ret = sys_arch_sem_wait(&lwip_netif_up, 15000);
     if (ret == SYS_ARCH_TIMEOUT) {
@@ -183,12 +182,10 @@ int lwip_bringup(void)
 
 void lwip_bringdown(void)
 {
+    // Disconnect from the network
     dhcp_release(&lwip_netif);
     dhcp_stop(&lwip_netif);
-
-    eth_arch_disable_interrupts();
     lwip_ip_addr[0] = '\0';
-    lwip_mac_addr[0] = '\0';
 }