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.
The hwtimer_wait test was tortured with the
following script which ran several hours.
----
make clean all-debug
while :
do
date
./bin/native/hwtimer_wait.elf > log &
pid=$!
sleep 20
success=$(cat log|grep success)
if [ "$success" != "success" ]
then
date
echo "BUG"
exit
fi
kill $pid
done
----
Closes#715.
