setup_default_uart() function

Set up the default UART and assign it to the default GPIOs By default this will use UART 0, with TX to pin GPIO 0, RX to pin GPIO 1, and the baudrate to 115200 Calling this method also initializes stdin/stdout over UART if the pico_stdio_uart library is linked. Defaults can be changed using configuration defines, PICO_DEFAULT_UART_INSTANCE, PICO_DEFAULT_UART_BAUD_RATE PICO_DEFAULT_UART_TX_PIN PICO_DEFAULT_UART_RX_PIN