Fixed NVIC_priority, added navigation task, defining navagation structure
initialized nav struct
deleted unused nav_init function
Merge pull request #125 from ming6842/add_OS_for_test

update flight control
flight control: fix double initilization issue
user can disable/enable some devices
Feed real attitude(include compass) and altitude data
Use reasonalbe timeout value
Merge branch 'mavlink_new_board' of https://github.com/moon-bear/firmware into os_dev

Conflicts:
	program/main.c
Nano lib should be linked after compiling
Use antoher method to send string
FIXME: a bug in printf
Because we cannot use printf in FreeRTOS now. I use alternative way to combine sprintf and our usart8_send_string
Calculate the period directly
Finish system time which bases on main control loop rate
Fix duplicate definition
Move test code out
Merge pull request #127 from moon-bear/mavlink_real_data

Mavlink real data
increse uart rx queue size, let rx task have higher priority
Increase the rate of transmitting heartbeat, slower down others
Let mavlink task parsing speed be more quick
1.use new system_time
2.timeout to be 1
3. fix timeout decision problem((cur_time - start_time) == TIMEOUT_CNT)
Added waypoint/loiter time