ROMFS: added mixer for tailsitter simulation

- simulated tailsitter needs a virtual elevator since we cannot simulate
elevons yet (liftDrag plugin does not model longitudinal moment Cm)

Signed-off-by: Roman <bapstroman@gmail.com>

ROMFS/px4fmu_common/mixers/quad_x_vtol_sim.main.mix

+Mixer for the gazebo tailsitter model
+=====================================
+
+R: 4x 10000 10000 10000 0
+
+Z:
+
+# left elevon
+M: 1
+O:      10000  10000      0 -10000  10000
+S: 1 0  10000  10000      0 -10000  10000
+
+# right elevon
+M: 1
+O:       10000   10000    0 -10000  10000
+S: 1 0   -10000   -10000    0 -10000  10000
+
+# mixer for the virtual elevator
+M: 1
+O:       10000   10000   0 -10000  10000
+S: 1 1   -10000   -10000    0 -10000  10000

posix-configs/SITL/init/ekf2/tailsitter

@@ -60,7 +60,7 @@ mc_pos_control start
 mc_att_control start
 fw_pos_control_l1 start
 fw_att_control start
-mixer load /dev/pwm_output0 ROMFS/px4fmu_common/mixers/quad_x_vtol.main.mix
+mixer load /dev/pwm_output0 ROMFS/px4fmu_common/mixers/quad_x_vtol_sim.main.mix
 mavlink start -x -u 14556 -r 2000000
 mavlink start -x -u 14557 -r 2000000 -m onboard -o 14540
 mavlink stream -r 80 -s POSITION_TARGET_LOCAL_NED -u 14556