Switching priorities of mirror defaults (which mirror default is used from...

Switching priorities of mirror defaults (which mirror default is used from which previously set mirror default etc.) to the way it was intendet to be.

If not building a derivative with two seperate repositories,
users should only need to bother about setting the
--mirror*|LB_MIRROR* options, the --parent-mirror*|LB_PARENT_MIRROR*
does get set automatically correctly now.