yocto: introduce optional dependencies before per-recipe sysroot

[lucaceresoli]

The "Per-recipe sysroot" slides introduce the per-recipe sysroot but not the main motivation it exists for, i.e. reproducible builds in presence of automatic optional dependencies in configuration scripts.

To me this takes a significant time in a verbal explanation, which would be more effective with a supporting slide.

Add a slide between "Sysroot" and "Per-recipe sysroot" with an explanatory example. E.g. a web server program optionally depending on an encryption library to enable https.

TODO: find a good example, simple and intuitive (looking at existing PACKAGECONFIG values is probably a good starting point)