NvtFly 1.42 Aug-4-2015
Improvements over 1.4:
1) Firmware auto select Baro(ms5611/bmp280) on board(T.L. Shen)
2) Support 100 usec system tick(T.L. Shen)
3) Simplify the pwm driver for effecency(C.P. Lu)  
4) Fly attitude stable control(T.L. Shen)
	a) Update default PID
	b) Change sensor filter frequency from 188Hz to 42Hz
5) Support brushed DC motor
6) Add firmware version information
7) Add Board version
8) Support HEX6X frame

Bug Fixes / Parameter changes:
- fix perpeller auto spin in a few minutes when user turn off remote controller and copter is under dis-arm state(T.L. Shen)
- fix MAG calbration QC error(T.L. Shen)
- fix brushed DC motor spin when pwm init 
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