Add WITH_REPRODUCIBLE_BUILD src.conf(5) knob to disable kernel metadata

[emaste]

Summary

Continuation of D4347 / rS310112

[jhb]

Do you envision this only being a kernel option or to later affect things in userland builds as well? If the latter, perhaps adjust the comment to be more generic (can reference the kernel bits as an example in that case).

[emaste]

In D8809#182357, @jhb wrote:

Do you envision this only being a kernel option or to later affect things in userland builds as well?

Userland is generally reproducible without an option. I do expect to extend this to boot loaders as well, and will update the comment to be general then. I didn't want to use a generic comment initially since it's not yet true.