main: Fix busy loop with reading

main: Fix busy loop with reading

As the baud.c opens the serial device with O_NBLOCK flag, read will
just return immediately when there is nothing to read. That basically
makes the read loop a a busy loop, consuming one CPU core fully at all
times.

Fix the problem by using epoll to wait until something becomes
available on the serial port to read.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>