driver: add ability to customize the line separator in drivers using ConsoleProtocol

[flxzt]

Description

This adds the ability to customize the line separator in drivers that implement ConsoleProtocol (through the ConsoleExpectMixin class).

These are:

• SerialDriver
• ExternalConsoleDriver
• QemuDriver

It is useful when certain systems expect a \r\n instead of the usual \n .

Checklist

• [x] Documentation for the feature
• [x] Tests for the feature
• [x] The arguments and description in doc/configuration.rst have been updated
• [x] PR has been tested