Allow specifying configuration common to all targets

A reserved target name like `all` could make sense to allow the user to define configuration common to all targets. This way, if all targets share some configuration, say, some image or post_build command, the user is not required to input the same configuration multiple times.

Start by tracing how target configuration is represented and merged, using the image and post_build examples. Determine how a reserved `all` target would share configuration with every target; done means users can provide that configuration once, but no files or tests are named.

Written by the indexing model from the issue text.