HAL_Linux: improved help output and errors for udp and bcast

libraries/AP_HAL_Linux/HAL_Linux_Class.cpp

@@ -232,7 +232,9 @@ void _usage(void)
     printf("\t                  -B /dev/ttyS1\n");
     printf("\t-tcp:             -C tcp:192.168.2.15:1243:wait\n");
     printf("\t                  -A tcp:11.0.0.2:5678\n");
-    printf("\t                  -A udp:11.0.0.2:5678\n");
+    printf("\t                  -A udp:11.0.0.2:14550\n");
+    printf("\t                  -A udp:11.0.0.255:14550:bcast\n");
+    printf("\t                  -A udpin:0.0.0.0:14550\n");
     printf("\t-custom log path:\n");
     printf("\t                  --log-directory /var/APM/logs\n");
     printf("\t                  -l /var/APM/logs\n");

libraries/AP_HAL_Linux/UARTDriver.cpp

@@ -312,6 +312,10 @@ void UARTDriver::_udp_start_connection(void)
  */
 void UARTDriver::_udpin_start_connection(void)
 {
+    bool bcast = (_flag && strcmp(_flag, "bcast") == 0);
+    if (bcast) {
+        AP_HAL::panic("Can't combine udpin with bcast");
+    }
     _device = new UDPDevice(_ip, _base_port, false, true);
     _connected = _device->open();
     _device->set_blocking(false);