AP_RPM: add explicit RPM_TYPE_SITL

Old behaviour is that anything that wasn't claimed (or failed to
allocate) would become a SITL RPM sensor.  This meant we always had at
least one RPM sensor which was allocated but never provided valid data.

New behaviour is that if you want a SITL RPM sensor you have to set type
to 10.

libraries/AP_RPM/AP_RPM.cpp

@@ -132,7 +132,7 @@ void AP_RPM::init(void)
             drivers[i] = new AP_RPM_HarmonicNotch(*this, i, state[i]);
         }
 #if CONFIG_HAL_BOARD == HAL_BOARD_SITL
-        if (drivers[i] == nullptr) {
+        if (type == RPM_TYPE_SITL) {
             drivers[i] = new AP_RPM_SITL(*this, i, state[i]);
         }
 #endif

libraries/AP_RPM/AP_RPM.h

@@ -41,7 +41,10 @@ public:
         RPM_TYPE_PWM     = 1,
         RPM_TYPE_PIN     = 2,
         RPM_TYPE_EFI     = 3,
-        RPM_TYPE_HNTCH   = 4
+        RPM_TYPE_HNTCH   = 4,
+#if CONFIG_HAL_BOARD == HAL_BOARD_SITL
+        RPM_TYPE_SITL   = 10,
+#endif
     };
 
     // The RPM_State structure is filled in by the backend driver