Add a user defined rules/config file not tracked by git

[Staphylo]

- Why I did it

This change introduces the rules/config.user file.
Since this path is tracked by .gitignore, it provides a mean to change
the configuration of a sonic build without dirtying the git tree.

- How I did it

This configuration gets loaded after rules/config as a mean to override default values.
Thus it allows the default configuration to grow with new default values without having
to track each and single one of them when one generates a configuration.
If the rules/config.user configuration file does not exists this step is ignored.

- How to verify it

Here is an example of the use of rules/config.user

echo DEFAULT_PASSWORD=password > rules/config.user

You should now see the new password being displayed when running a make operation.
Running git status should not show any files in the dirty section nor in the not staged section.

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006

- Description for the changelog

Add a user defined rules/config.user file not tracked by git

[jleveque]

@Staphylo: Can you please add a bit more information to the description as to how the rules/config.user is used?

[Staphylo]

Thanks for the review Joe.
I just updated the description. Let me know if that's enough.

[jleveque]

Looks good to me.