Plane: build AFS (formerly known as OBC) failsafe on Pixhawk

this gives an advanced failsafe system which may be useful for other
than OBC use

ArduPlane/Parameters.pde

@@ -1073,7 +1073,9 @@ const AP_Param::Info var_info[] PROGMEM = {
 #endif
 
 #if OBC_FAILSAFE == ENABLED
-    GOBJECT(obc,  "FS_", APM_OBC),
+    // @Group: AFS_
+    // @Path: ../libraries/APM_OBC/APM_OBC.cpp
+    GOBJECT(obc,  "AFS_", APM_OBC),
 #endif
 
 #if AP_AHRS_NAVEKF_AVAILABLE

ArduPlane/config.h

@@ -138,7 +138,6 @@
 #endif
 #endif
 
-
 //////////////////////////////////////////////////////////////////////////////
 //////////////////////////////////////////////////////////////////////////////
 // RADIO CONFIGURATION
@@ -488,8 +487,12 @@
 
 // OBC Failsafe enable
 #ifndef OBC_FAILSAFE
+#if HAL_CPU_CLASS >= HAL_CPU_CLASS_75
+ # define OBC_FAILSAFE ENABLED
+#else
  # define OBC_FAILSAFE DISABLED
 #endif
+#endif
 
 #ifndef SERIAL_BUFSIZE
  # define SERIAL_BUFSIZE 512