[autopilot] compiling generated AP for fixedwing (#2055)

* [autopilot] compiling generated AP for fixedwing

* update some old variable names for autopilot mode

* prevent oscillation on the ground with real aircraft

* add guidance loop to 'new' control

all this guidance code should really be better factorized

* fix rotorcraft autopilot for new API

* read guidance RC if needed

* use generated autopilot as soon as an autopilot file is defined

* autopilot node at airframe, firmware or target level

The autopilot can be specific to one of the firmware, or one of the
target.
At the moment, it will fail at runtime if more than one autopilot in the
same airframe since server don't know which target is in use.

sw/misc/rctx/main_rctx.c

@@ -177,7 +177,7 @@ void event_task_rctx( void) {
 #ifdef USE_RCTX_MODE_SWITCH
     // TODO: set rxtx_mode from GPIO connected switch (e.g. I2C pins)
 #else
-    rctx_mode = PPRZ_MODE_OF_PULSE(rc_values[RADIO_MODE], 0) & 3;
+    rctx_mode = AP_MODE_OF_PULSE(rc_values[RADIO_MODE], 0) & 3;
 #endif
 
     rctx_mode |= rctx_under_voltage << 2;