prevent dhcp from starting when netif link is down (only when LWIP_DHCP_CHECK_LINK_UP==1, which is disabled by default for compatibility reasons)

src/include/lwip/dhcp.h

@@ -161,9 +161,9 @@ void dhcp_fine_tmr(void);
 #define DHCP_REBINDING    4
 #define DHCP_RENEWING     5
 #define DHCP_SELECTING    6
-#define DHCP_INFORMING    7
+/* not yet implemented #define DHCP_INFORMING    7*/
 #define DHCP_CHECKING     8
-#define DHCP_PERMANENT    9
+/* not yet implemented #define DHCP_PERMANENT    9*/
 #define DHCP_BOUND        10
 /** not yet implemented #define DHCP_RELEASING 11 */
 #define DHCP_BACKING_OFF  12

src/include/lwip/opt.h

@@ -751,6 +751,16 @@
 #define DHCP_DOES_ARP_CHECK             ((LWIP_DHCP) && (LWIP_ARP))
 #endif
 
+/**
+ * LWIP_DHCP_CHECK_LINK_UP==1: dhcp_start() only really starts if the netif has
+ * NETIF_FLAG_LINK_UP set in its flags. As this is only an optimization and
+ * netif drivers might not set this flag, the default is off. If enabled,
+ * netif_set_link_up() must be called to continue dhcp starting.
+ */
+#ifndef LWIP_DHCP_CHECK_LINK_UP
+#define LWIP_DHCP_CHECK_LINK_UP         0
+#endif
+
 /**
  * LWIP_DHCP_BOOTP_FILE==1: Store offered_si_addr and boot_file_name.
  */