Add overridden distro configuration for poky

[ycongal-smile]

• Add function to get the overridden distro configuration if it exists
• Add rule in schema to allow to add a new entry in menu
• Add test case for an overridden distro

This is needed for a somewhat exotic setup : When you clone the individual Yocto repo instead of the poky.git combo-layer (Which is easier in order to contribute upstream). In this case the cloned repos can't have the same layout as poky.git.

Note: Ever is the author of the patches, I've reviewed them and will handle the PR from here (Ever has agreed. I hope that's OK)

[ycongal-smile]

Hi @cpb- @pboettch "gentle ping" on this PR. I'm afraid you missed it...? Any way I can help you review/merge this?

[ycongal-smile]

Thank you for merging :)