Ack timeout must be at least 2 seconds

While calculating ack timeout, we were ending up getting a random value
which may become less than 2 seconds. This is not allowed as per v1.0.2
specification.

To fix the issue we now take the random number from 0 to 2000 ms and
then add that to the fixed 2000 ms ack timeout value, guaranteeing a
value at least equal to 2000 ms.

features/lorawan/lorastack/phy/LoRaPHY.cpp

@@ -630,8 +630,7 @@ uint16_t LoRaPHY::get_maximum_frame_counter_gap()
 uint32_t LoRaPHY::get_ack_timeout()
 {
     uint16_t ack_timeout_rnd = phy_params.ack_timeout_rnd;
-    return (phy_params.ack_timeout
-            + get_random(-ack_timeout_rnd, ack_timeout_rnd));
+    return (phy_params.ack_timeout + get_random(0, ack_timeout_rnd));
 }
 
 uint32_t LoRaPHY::get_default_rx2_frequency()