initial commit
update.sh: Set PYTHON_TARGETS and PYTHON_SINGLE_TARGET to drop Python 2

Then rebuild everything that cares about those variables to reflect
the update.  The existing depclean will remove the no-longer-needed
Python 2.
more tweaks to update.sh including nice whitespace and some PYTHON_TARGETS autodetection
stage3-amd64-20140529.tar.xz