SYS_HITL: reserve value -1 for external HITL

ROMFS/px4fmu_common/init.d/airframes/4070_aerofc

@@ -70,7 +70,7 @@ then
 	param set SER_TEL2_BAUD 921600
 fi
 
-if param compare SYS_HITL 0
+if ! param greater SYS_HITL 0
 then
   tap_esc start -d /dev/ttyS0 -n 4
   usleep 300000

src/lib/systemlib/system_params.c

@@ -70,6 +70,11 @@ PARAM_DEFINE_INT32(SYS_AUTOCONFIG, 0);
  * or Simulation-In-Hardware (SIH) mode and not enable all sensors and checks.
  * When disabled the same vehicle can be flown normally.
  *
+ * Set to 'external HITL', if the system should perform as if it were a real
+ * vehicle (the only difference to a real system is then only the parameter
+ * value, which can be used for log analysis).
+ *
+ * @value -1 external HITL
  * @value 0 HITL and SIH disabled
  * @value 1 HITL enabled
  * @value 2 SIH enabled