Copter: rename FULL_IMU logging to IMU_FAST

This naming is consistent with the earlier ATTITUDE_FAST option and
clarifies that it's the rate that's increased rather than the content.

ArduCopter/ArduCopter.pde

@@ -1008,7 +1008,7 @@ static void fifty_hz_logging_loop()
 // should be run at the MAIN_LOOP_RATE
 static void full_rate_logging_loop()
 {
-    if (should_log(MASK_LOG_FULL_IMU)) {
+    if (should_log(MASK_LOG_IMU_FAST)) {
         DataFlash.Log_Write_IMU(ins);
     }
 }

ArduCopter/defines.h

@@ -261,7 +261,7 @@ enum FlipState {
 #define MASK_LOG_CAMERA                 (1<<15)
 #define MASK_LOG_WHEN_DISARMED          (1UL<<16)
 #define MASK_LOG_MOTBATT                (1UL<<17)
-#define MASK_LOG_FULL_IMU               (1UL<<18)
+#define MASK_LOG_IMU_FAST               (1UL<<18)
 #define MASK_LOG_ANY                    0xFFFF
 
 // DATA - event logging