serial_conn.py: use ^D aka \003 as write_trigger for login prompt

For system where a password is required to login the former solution
triggering with 'exit\n' leads to an endless loop.

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>
serial_conn.py: don't start read_until by sending '\n' unconditionally

There's no need to send '\n' in every call of read_until. Quite the
contrary this leads to problems answering login or password prompts.

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 #12 from DFE/b-fix-login

Login problems over serial connection