Merge pull request #1 from shengwen1997/master

Initial import
Disable "throttle off" function. It could stablize in starup now.
"Throttle off" means system will set the minimum rotations of motor.
This mechanism ensure the safety of flight. But we don't use remoter
now. So I remove it let the flihgt control could control its stablizatoin
in startup.
Import uVision project files
Enable PWM2(Timer4 CH2) to capture input PWM signal
In order to enable this funciton, I must disable
PWM2 output function.
Replace PWM input capture to normal input capture

Because PWM input capture need two channel, we need to use another way
to calculate high voltage level time.
Enable to measure pulse width in Timer4(CH1, CH2)

Discard old method to measure the pulse width. I  change the input
capture polarity when MCU enter the input capture interrupt. I also set
60Mhz time base for timer 4. This frequency will ensure the CCR will not
overflow when It still in the high level voltage. I test it with servo
tester. It works very well now.
Remove TIM2 CH1 CH2 PWM configuration
Reference to shengwen1997

Add Makefile & stm32_flash.ld
Consistent with the previous code of FLASH part
Fix source code of Makefile.
Add toolchain path in the Makefile
fix error
fix error
Fix misspell, and Makefile can target.
Enable TIM2 CH1 CH2 input capture mode

TIM2 CH1 is PWM3, TIM2 CH2 is PWM4.
Pin Config:
https://github.com/Hom19910422/QCopterFlightControl/wiki/Config
Remove TIM2 CH3 CH4 PWM Config

Because we still have TIM CH3 and CH4 in PWM config, It has some  time
base conflic problem. I decide I will not use TIM for PWM output in
future.
Rearrange initial configuration
Enable TIM2 CH1 and CH2 to capture PWM pulse
Enable TIM2 CH3 to capture PWM pulse

TODO: deal with signal lost and some interference
Add startup_stm32f4xx.s to support Makefile

Because startup_stm32f40xx.s have many bugs.