fcntl(fd, F_SETOWN, getpid()); doesn't seem to work on Linux
to get generate a signal when an event on the GPIO fd occurs.
So fall back to the same method as on OS X and call poll() in
a child process.
select() can not listen to POLLPRI events which are used by the
Kernel's GPIO API.
In preparation for that, rewrite async_read() to use poll() instead
of select().
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.
