codeload.py: improve serial line target response and error handling

[TG9541]

In serial mode codeload.py has always had difficulties with a target response that contained more than one LF (e.g. due to test output). In telnet mode it uses the Python telnet.expect() method which is more robust than Python serial.port.readline(). Python serial.port.read_until() should be more flexible.

Also error information is not as helpful as it should be.