add fetch_sources command

Simple function to retrieve all recipes for the packages included in the current manifest(s), check if they have do_configure (or at least do_patch or do_unpack) task, and run the task.

It is intended to be run after a bitbake quick build that leverages SSTATE cache (and thus does not fetch nor configure sources, because it reuses existing build artifacts), to fetch and configure sources (but without rebuilding them) in order to be able to match binary files with source files.

The tasks are run only on recipes that correspond to packages that are actually included in the final images: bitbake will automatically run all the other necessary tasks on build dependency recipes.