Generate $(SCRIPTOUT) during the build phase

[AlanRosenthal]

Summary

The only use of the boards::context was to generate $(SCRIPTOUT) that is used by the linker.

Instead of generating it at context phase, generate it via normal dependency specification. This PR also removes the board::context rule, as it is no longer required.

Impact

This work is part of the effort in #5205, which will clean up the context generation phase.

Testing

Verified build locally

[AlanRosenthal]

To make this PR easier to review, I moved a lot of the changes to #5496

[AlanRosenthal]

need to rebase/fix some issues, no need for review right now