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.
Makefile can work very well now
TODO: We should not compile all file. Some of them are useless for
us.(Reading uVision project file)
Merge to fboris