sys/uart_stdio: rename STDIO -> UART_STDIO_DEV, move to header

boards/arduino-mega2560/include/board.h

boards/iotlab-m3/include/board.h

boards/msb-430-common/board_init.c

boards/nucleo-f103/include/board.h

boards/telosb/board.c

boards/wsn430-common/board_init.c

boards/z1/board.c

boards/z1/include/board.h

drivers/include/ethos.h

sys/include/uart_stdio.h

sys/uart_stdio/uart_stdio.c

tests/driver_xbee/README.md

tests/periph_uart/main.c