introduce separate vars target_branch_iso and target_branch_pkg

Closed Bernhard Landauer requested to merge oberon-manjaro:target_branch into master

Just a suggestion: Although we already have option -b available, at times like now when we are building testing and stable ISOs and at the same time continue building packages against unstable, I think it is very convenient to be able to set target_branch separately for ISOs and pkgs in manjaro-tools.conf.

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