Add supports of UART0 to UART5 for beaglebone(black)

Add RT_USING_UART0 to RT_USING_UART5 in config.h:
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If want to use uart4, you can modify the file rtconfig.h with ```#define RT_USING_UART4```, and the device name of uart4 is 'uart4'.

Set the console default via UART0:
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It comes with ```#define RT_CONSOLE_DEVICE_NAME	"uart0"``` In rtconfig.h.