buildsystem: Call Buildtest with Clean Environment

[crasbe]

Contribution description

The builds for individual boards for the buildtest goal were not called with a clean environment, leading to the Make system building the binaries in the incorrect folder. For more details see #9742.

The change in the Makefile.include is necessary to avoid makefiles/board_alias.inc.mk printing a warning about the alias from native to native64, but other bad effects are possible from having the wrong BOARD defined.

Testing procedure

Outlined in #9742.

Issues/PRs references

Fixes #9742.

[maribu]

Nice one!

I wonder, though, if we should just remove the buildtest command. I have been using make generate-Makefile.ci in its place for years now. We could let buildtest just call that instead IMO, plus a warning that the Makefile.ci will be updated as side effect.

[mguetschow]

I wonder, though, if we should just remove the buildtest command.

FWIW, I didn't even know about buildtest up until now. If it adds no benefit over generate-Makefile.ci, I'd favor removing it.

[crasbe]

Closed in favor of #21359.