serial_io: separated read and write process

Makes testing easier and the program itself more flexible. Also increases the
likeness of this interface to serial_conn.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
serial_io: fundamental unit tests and debugging

Added tests for Object creation, a basic cmd() and 2 cases for the read_until
methods. The last method needed an additonal unit test, because start_strip and
end_strip should be tested separately.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>