build.sh:start_build: always build all `invariants' target packages.
build.sh:exec_start, vars/invariants_{digest,tarballs}.vars: move ${ARG_TARBALL} checks into invariants_{digest,tarballs}.
etc/build.usage: update targets list.
vars/build.vars:dist_minipix: renamed to `minipix'.
vars/build.vars:dist: split into {dist,invariants}.
vars/dist_gitref.vars: renamed to vars/invariants_gitref.vars.