preflight checks: add check for distance sensor (via parameter)

src/lib/systemlib/system_params.c

@@ -218,6 +218,21 @@ PARAM_DEFINE_INT32(SYS_HAS_MAG, 1);
  */
 PARAM_DEFINE_INT32(SYS_HAS_BARO, 1);
 
+/**
+ * Control the number of distance sensors on the vehicle
+ *
+ * If set to the number of distance sensors, the preflight check will check
+ * for their presence and valid data publication. Disable with 0 if no distance
+ * sensor present or to disbale the preflight check.
+ *
+ * @reboot_required true
+ *
+ * @group System
+ * @min 0
+ * @max 4
+ */
+PARAM_DEFINE_INT32(SYS_HAS_NUM_DIST, 0);
+
 /**
  * Enable factory calibration mode
  *