Ability to specify build depends that are used by all elements

Background

I'm seeing projects where all elements are having a build dependency on the same element(s); typically the base platform and toolchain. Having a way to define this common dependency in project.conf is more convenient/less verbose.

Task description

Add a configuration option to project.conf When expanding element dependencies, start with what is configured, then extend with the explicit depends of the element

Acceptance Criteria

No longer having to specify common dependencies in all elements.