Use hardware timer and binary semaphore to control precice duration
This commit still have not done verify yet. In this commit, I did
1)use this method to keep the frequency of flight control task be 4kHz
2)disable tim10(we do not use it now, prevent some isr issues)
3)use led 2 to debug timer interrupt
Remove the PC9 timer initialization
Merge branch 'fix_flight_control_freq' of https://github.com/fboris/firmware into os_dev
Enable the ground station communication task
Merge pull request #146 from moon-bear/fix_flight_control_freq

Use timer interrupt to ensure the flight control excute frequence is about 4,000hz (Not being flight test yet)
Merge branch 'os_dev' of https://github.com/moon-bear/firmware into mavlink