Separate the partitioning part of confine-system into confine-parted

This package will be obsolete and not installed on future images using the
upcoming boot/upgrade process
Use global vars with ':=' in the Makefile
Add lazy implementation of the confine-init-system package
Add new SYSTEM make variable to define what confine system to build

The one build now is still the default, called "regular". "init" and "node"
will be the ones used in the new boot system, the former being the one that
manages the many node images that were built with the "node" option.