MP-3: Image build process

[cloudtoad]

Placeholder indicating I intend to create the MP-3 draft proposal...

Will cover custom kernel building, and building various images from scratch in an automated way.

[Mierdin]

👍 Be sure to link to this issue in the PR, and don't be afraid to open a draft PR as early as possible, even if you just throw a two-liner skeleton markdown file up there.