AP_HAL_SITL: Added support for blimp simulator

libraries/AP_HAL_SITL/SITL_State.h

@@ -69,7 +69,8 @@ public:
         ArduCopter,
         Rover,
         ArduPlane,
-        ArduSub
+        ArduSub,
+        Blimp
     };
 
     int gps_pipe(uint8_t index);

libraries/AP_HAL_SITL/SITL_cmdline.cpp

@@ -35,6 +35,7 @@
 #include <SITL/SIM_Scrimmage.h>
 #include <SITL/SIM_Webots.h>
 #include <SITL/SIM_JSON.h>
+#include <SITL/SIM_Blimp.h>
 #include <AP_Filesystem/AP_Filesystem.h>
 
 #include <signal.h>
@@ -157,6 +158,7 @@ static const struct {
     { "scrimmage",          Scrimmage::create },
     { "webots",             Webots::create },
     { "JSON",               JSON::create },
+    { "blimp",              Blimp::create },
 };
 
 void SITL_State::_set_signal_handlers(void) const