udp: Add support for SO_TIMESTAMP

Adds support for timestamping received UDP packets, either in
hardware or in kernel. Builds on the existing support of SO_TIMESTAMP
for SocketCAN.

Implementation uses CLOCK_REALTIME for timestamping to match the
behavior of Linux. This could be made configurable in future if needed.

include/nuttx/net/netdev.h

@@ -442,6 +442,18 @@ struct net_driver_s
   struct netdev_statistics_s d_statistics;
 #endif
 
+#if defined(CONFIG_NET_TIMESTAMP)
+  /* Reception timestamp of packet being currently processed.
+   * If CONFIG_ARCH_HAVE_NETDEV_TIMESTAMP is true, the timestamp is provided
+   * by hardware driver. Otherwise it is filled in by kernel when packet
+   * enters ipv4_input or ipv6_input.
+   *
+   * The timestamp is in CLOCK_REALTIME.
+   */
+
+  struct timespec d_rxtime;
+#endif
+
   /* Application callbacks:
    *
    * Network device event handlers are retained in a 'list' and are called