Copter: add option to disable DRIFT flight mode

Saves about 1kB of space

ArduCopter/APM_Config.h

@@ -24,6 +24,7 @@
 //#define SPRAYER               DISABLED            // disable the crop sprayer feature (two ESC controlled pumps the speed of which depends upon the vehicle's horizontal velocity)
 //#define WINCH_ENABLED         DISABLED            // disable winch support
 //#define MODE_AUTO_ENABLED     DISABLED            // disable auto mode support
+//#define MODE_DRIFT_ENABLED    DISABLED            // disable drift mode support
 //#define MODE_LOITER_ENABLED   DISABLED            // disable loiter mode support
 //#define MODE_POSHOLD_ENABLED  DISABLED            // disable poshold mode support
 //#define MODE_SMARTRTL_ENABLED DISABLED            // disable smartrtl mode support

ArduCopter/Copter.h

@@ -964,7 +964,9 @@ private:
 #endif
     ModeBrake mode_brake;
     ModeCircle mode_circle;
+#if MODE_DRIFT_ENABLED == ENABLED
     ModeDrift mode_drift;
+#endif
     ModeFlip mode_flip;
     ModeGuided mode_guided;
     ModeLand mode_land;

ArduCopter/config.h

@@ -273,6 +273,12 @@
 # define MODE_AUTO_ENABLED ENABLED
 #endif
 
+//////////////////////////////////////////////////////////////////////////////
+// Drift - fly vehicle in altitude-held, coordinated-turn mode
+#ifndef MODE_DRIFT_ENABLED
+# define MODE_DRIFT_ENABLED ENABLED
+#endif
+
 //////////////////////////////////////////////////////////////////////////////
 // Loiter mode - allows vehicle to hold global position
 #ifndef MODE_LOITER_ENABLED

ArduCopter/mode.cpp

@@ -78,9 +78,11 @@ Copter::Mode *Copter::mode_from_mode_num(const uint8_t mode)
             ret = &mode_rtl;
             break;
 
+#if MODE_DRIFT_ENABLED == ENABLED
         case DRIFT:
             ret = &mode_drift;
             break;
+#endif
 
 #if MODE_SPORT_ENABLED == ENABLED
         case SPORT: