Copter: add option to disable POSHOLD mode

Saves about 4k of Flash

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_POSHOLD_ENABLED  DISABLED            // disable poshold mode support
 
 // features below are disabled by default on all boards
 //#define CAL_ALWAYS_REBOOT                         // flight controller will reboot after compass or accelerometer calibration completes

ArduCopter/Copter.h

@@ -969,7 +969,9 @@ private:
     ModeGuided mode_guided;
     ModeLand mode_land;
     ModeLoiter mode_loiter;
+#if MODE_POSHOLD_ENABLED == ENABLED
     ModePosHold mode_poshold;
+#endif
     ModeRTL mode_rtl;
 #if FRAME_CONFIG == HELI_FRAME
     ModeStabilize_Heli mode_stabilize;

ArduCopter/config.h

@@ -273,6 +273,12 @@
 # define MODE_AUTO_ENABLED ENABLED
 #endif
 
+//////////////////////////////////////////////////////////////////////////////
+// Position Hold - enable holding of global position
+#ifndef MODE_POSHOLD_ENABLED
+# define MODE_POSHOLD_ENABLED ENABLED
+#endif
+
 //////////////////////////////////////////////////////////////////////////////
 // RADIO CONFIGURATION
 //////////////////////////////////////////////////////////////////////////////

ArduCopter/mode.cpp

@@ -94,9 +94,11 @@ Copter::Mode *Copter::mode_from_mode_num(const uint8_t mode)
             break;
 #endif
 
+#if MODE_POSHOLD_ENABLED == ENABLED
         case POSHOLD:
             ret = &mode_poshold;
             break;
+#endif
 
         case BRAKE:
             ret = &mode_brake;