xbee: add packet filtering to emulate non-transitive network.

When debugging multihop wireless network, it is useful to emulate non-transitive
network, that is, node A can communicate with B and B can communicate with C,
but A cannot communicate with C directly.

If `XBEE_DENIED_ADDRESSES`, which is an array of XBee long addresses, is
defined, packets from those addresses are dropped silently.

Example:
CFLAGS += "-DXBEE_DENIED_ADDRESSES={ 0x02, 0x12, 0x34, 0x56, 0x78, 0x9a, 0xbc, 0xde, 0x02, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11, 0x11 }"

drivers/xbee/xbee.c

@@ -743,6 +743,23 @@ static void _isr_event(gnrc_netdev_t *netdev, uint32_t event_type)
         addr_len = 8;
     }
 
+#ifdef XBEE_DENIED_ADDRESSES
+    if (addr_len == 8) {
+        uint8_t denied_addresses[] = XBEE_DENIED_ADDRESSES;
+        for (size_t i = 0; i < sizeof(denied_addresses) / 8; i++) {
+            if (memcmp(&(dev->rx_buf[1]),
+                       &denied_addresses[i * 8],
+                       addr_len) == 0) {
+                dev->rx_count = 0;
+
+                DEBUG("xbee: dropping denied packet\n");
+
+                return;
+            }
+        }
+    }
+#endif
+
     /* check checksum for correctness */
     for (int i = 0; i < dev->rx_limit; i++) {
         cksum += dev->rx_buf[i];