uart0 functionality is removed by #3164. This patch implements periph/uart,
rather than deprecated uart0, using /dev/tty.
To use with netdev2_tap simultaneously, this patch adds asynchronus read system
and modifies netdev2_tap to use it.
A TTY device is specified on command line with -c (COM) option, since -t was
used by the old implementation.
This patch also implements empty GPIO driver needed by the xbee driver.