message: tools: invert version logic, assume v1.11/later
  Instead of having an env variable for PX4 v1.11 and later, it makes more
  sense having to set it for older versions and assume v1.11 and later by
  default.
diff: @@ -10,10 +10,7 @@ fi
  sitl_model=$1
  echo "SITL model: $sitl_model"
  -if [ "$PX4_VERSION" = "v1.11" ]; then
  sitl_world="empty"
  - echo "SITL world: $sitl_world"
  -fi
  set -e
  @@ -60,21 +57,22 @@ pushd .
  cd $px4_firmware_dir/build/px4_sitl_default/tmp/rootfs
  # And run
  -if [ "$PX4_VERSION" = "v1.11" ]; then
  +if [ "$PX4_VERSION" = "v1.9" ] || [ "$PX4_VERSION" = "v1.10" ]; then
  $px4_firmware_dir/Tools/sitl_run.sh \
  $px4_firmware_dir/build/px4_sitl_default/bin/px4 \
  none \
  gazebo \
  $sitl_model \
  - $sitl_world \
  $px4_firmware_dir \
  $px4_firmware_dir/build/px4_sitl_default &
  else
  + # 1.11 onwards requires the world argument
  $px4_firmware_dir/Tools/sitl_run.sh \
  $px4_firmware_dir/build/px4_sitl_default/bin/px4 \
  none \
  gazebo \
  $sitl_model \
  + $sitl_world \
  $px4_firmware_dir \
  $px4_firmware_dir/build/px4_sitl_default &
  fi
  