Enable USART0 as console instead of DBGU port on ATMEL sam9x25 cpu board

sam9x25-ek开发板自带源码，keil编辑环境，是学习sam9x25的参考代码。-The development board comes with source of sam9x25-ek, keil editing environment is the learning sam9x25 reference code.

This user guide for Evaluation Kit & describes the development and debugging capabilities running on an AT91SAM9 ARM- based Embedded MPU as SAM9G15,SAM9G25,SAM9x25,SAM9G35,SAM9X35

at91sam 9x25 datasheet 对学习sam9x25 帮助非常大，首先要了解各部件的关系和具体应用。(AT91SAM 9x25 datasheet is very helpful for learning sam9x25, first of all, we should understand the relationship and specific application of each component.)