buildelement.py: Remove secret undocumented command lists

Originally this was created with also bootstrap-commands and test-commands but these were never documented or used.

For bootstrap-commands, these originated in baserock but are basically a part of the configure-commands stage, prepending commands to configure-commands is more suitable here since we now have the prepend/append list directives.

For test-commands, these were never used and it's unclear at this time if it's the correct place for it. It would be interesting to implement test-commands as a separate operation which can run in parallel with reverse dependency builds (no need to block the build of a reverse dependency on failing tests, we can still fail the build as a whole based on a failing test without blocking builds).