serial_conn.py: use \n as write_trigger for login prompt

The decision to use ^D as login write_trigger was not really good.
It seems that depending on the buffer management the trigger string
may be suspended for a long time. This is fixed by using \n instead.

Signed-off-by: Steffen Sledz <sledz@dresearch-fe.de>
Acked-by: Sascha Dierberg <dierberg@dresearch-fe.de>
Acked-by: Mario Schuknecht <schuknecht@dresearch-fe.de>
Merge pull request #14 from DFE/b-fix2-login

Login problems over serial connection (Part II)