Add ignition gazebo support for PX4 Software-In-The-Loop simulations

This commit adds SITL support for ignition gazebo. Ignition gazebo is a replacement for the "classic" gazebo for future applications.

The simulation can be run as the following
```
make px4_sitl ignition
```

Tools/setup_ignition.bash

@@ -0,0 +1,23 @@
+#!/bin/bash
+#
+# Setup environment to make PX4 visible to Gazebo.
+#
+# Note, this is not necessary if using a ROS catkin workspace with the px4
+# package as the paths are exported.
+#
+# License: according to LICENSE.md in the root directory of the PX4 Firmware repository
+
+if [ "$#" != 2 ]; then
+    echo -e "usage: source setup_gazebo.bash src_dir build_dir\n"
+    return 1
+fi
+
+SRC_DIR=$1
+BUILD_DIR=$2
+
+# setup Gazebo env and update package path
+export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:${SRC_DIR}/build/px4_sitl_default/build_ign_gazebo
+export IGN_GAZEBO_SYSTEM_PLUGIN_PATH=$IGN_GAZEBO_SYSTEM_PLUGIN_PATH:${SRC_DIR}/build/px4_sitl_default/build_ign_gazebo
+export IGN_GAZEBO_RESOURCE_PATH=$IGN_GAZEBO_RESOURCE_PATH:${SRC_DIR}/Tools/simulation-ignition/models
+
+echo -e "LD_LIBRARY_PATH $LD_LIBRARY_PATH"

Tools/sitl_run.sh

@@ -177,6 +177,10 @@ elif [ "$program" == "gazebo" ] && [ ! -n "$no_sim" ]; then
 		echo "You need to have gazebo simulator installed!"
 		exit 1
 	fi
+elif [ "$program" == "ignition" ] && [ -z "$no_sim" ]; then
+	echo "Ignition Gazebo"
+	source "$src_path/Tools/setup_ignition.bash" "${src_path}" "${build_path}"
+	ign gazebo -r "${src_path}/Tools/simulation-ignition/worlds/ignition.world"&
 elif [ "$program" == "flightgear" ] && [ -z "$no_sim" ]; then
 	echo "FG setup"
 	cd "${src_path}/Tools/flightgear_bridge/"