read_data: Handle the input data more gracefully

read_data: Handle the input data more gracefully

The serial port is giving the data to us very slowly, perhaps only a
few bytes a time. Therefore we need to take special care in order to
not handle incomplete log lines.

Add a new function to read one complete line of data from the
device. Once the complete line is read, write it to the logfile (if
one is open) and then also to the console.

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