Use DMA circular mode to transimit data through USART
Merge branch 'master' into dma

Conflicts:
	main.c
Support simple checksum
This checksum is very similar to NMEA checksum.
Define TxBuffer size
Seperate USART1 initialization and enable part
Enable DMA1 channel 4 intterupt
TODO: double buffer with dma usart
Use double buffer to prevent unexpected value
This method could prevent the situation that dma move the data, but CPU try to access the memory.
Ref:http://ntn314.blog.163.com/blog/static/161743584201042593319590/