experimental/gyro_fft: improve peak detection, add start parameter

- add new parameter `IMU_GYRO_FFT_EN` to start
 - add 75% overlap in buffer to increase FFT update rate
 - space out FFT calls (no more than 1 per cycle)
 - increase `IMU_GYRO_FFT_MIN` default
 - decrease main stack usage

ROMFS/px4fmu_common/init.d/rcS

@@ -526,6 +526,11 @@ else
 		bst start -X
 	fi
 
+	if param compare IMU_GYRO_FFT_EN 1
+	then
+		gyro_fft start
+	fi
+
 	#
 	# Optional board supplied extras: rc.board_extras
 	#