gnrc_dhcpv6_client: fix for SLIP link-layer

In a point-to-point serial connection, we don't have L2 addresses.
Set the link type & packet length accordingly and we will get a
prefix from KEA.

sys/include/net/arp.h

@@ -32,6 +32,7 @@ extern "C" {
  * @{
  */
 #define ARP_HWTYPE_ETHERNET (1U)        /**< Ethernet */
+#define ARP_HWTYPE_SERIAL   (20U)       /**< Serial Line */
 #define ARP_HWTYPE_EUI64    (27U)       /**< EUI-64 */
 /** @} */
 

sys/net/gnrc/application_layer/dhcpv6/client.c

@@ -52,6 +52,11 @@ unsigned dhcpv6_client_get_duid_l2(unsigned iface, dhcpv6_duid_l2_t *duid)
     }
     else {
         switch (netif->device_type) {
+            case NETDEV_TYPE_SLIP:
+                duid->l2type = byteorder_htons(ARP_HWTYPE_SERIAL);
+                /* L2 address length is 0 */
+                res = 0;
+                break;
             case NETDEV_TYPE_ETHERNET:
             case NETDEV_TYPE_BLE:
             case NETDEV_TYPE_ESP_NOW: