AP_HAL: ensure all UARTDriver implementations have receive_time_constraint_us

this fixes NMEA over network ports

libraries/AP_HAL/UARTDriver.cpp

@@ -152,3 +152,14 @@ bool AP_HAL::UARTDriver::discard_input()
     }
     return _discard_input();
 }
+
+/*
+  default implementation of receive_time_constraint_us() will be used
+  for subclasses that don't implement the call (eg. network
+  sockets). Best we can do is to use the current timestamp as we don't
+  know the transport delay
+ */
+uint64_t AP_HAL::UARTDriver::receive_time_constraint_us(uint16_t nbytes)
+{
+    return AP_HAL::micros64();
+}

libraries/AP_HAL/UARTDriver.h

@@ -137,10 +137,8 @@ public:
       This takes account of the baudrate of the link. For transports
       that have no baudrate (such as USB) the time estimate may be
       less accurate.
-
-      A return value of zero means the HAL does not support this API
      */
-    virtual uint64_t receive_time_constraint_us(uint16_t nbytes) { return 0; }
+    virtual uint64_t receive_time_constraint_us(uint16_t nbytes);
 
     virtual uint32_t bw_in_bytes_per_second() const {
         return 5760;