Add VTOL + tailsitter mixer

ROMFS/px4fmu_common/mixers/quad_+_vtol.main.mix

@@ -0,0 +1,28 @@
+Mixer for Tailsitter with + motor configuration and elevons
+===========================================================
+
+This file defines a single mixer for tailsitter with motors in X configuration.  All controls
+are mixed 100%.
+
+R: 4+ 10000 10000 10000 0
+
+# mixer for the elevons
+M: 2
+O:      10000  10000      0 -10000  10000
+S: 1 0  10000  10000      0 -10000  10000
+S: 1 1  10000  10000      0 -10000  10000
+
+M: 2
+O:       10000   10000    0 -10000  10000
+S: 1 0   10000   10000    0 -10000  10000
+S: 1 1  -10000  -10000    0 -10000  10000
+
+# mixer for canard surface
+M: 2
+O:       10000   10000    0 -10000  10000
+S: 1 1  -10000  -10000    0 -10000  10000
+
+# mixer for rudder
+M: 2
+O:       10000   10000    0 -10000  10000
+S: 1 2  -10000  -10000    0 -10000  10000