Cache SerialPort in SerialLoggerFactory

[oblaise]

Description

Only create SerialPort in SerialLoggerFactory if it wasn't already created.

Motivation and Context

Avoid exception if SerialLoggerFactory.CreateLogger is called multiple times enabling multiple loggers on the same serial port.

How Has This Been Tested?

N/A

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Config and build (change in the configuration and build system, has no impact on code or features)
• Dependencies (update dependencies and changes associated, has no impact on code or features)
• Unit Tests (work on Unit Tests, has no impact on code or features)
• Documentation (changes or updates in the documentation, has no impact on code or features)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[dnfadmin]

All CLA requirements met.

[josesimoes]

LGMT! Thanks for the nice improvement! 👍🏻

[Ellerbach]

Thanks a lot for this improvement!

[nfbot]

@oblaise thank you again for your contribution! 🙏😄

.NET nanoFramework it's all about community involvement and no contribution is too small.
We would like to invite you to join the project Contributors list.

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