Commit 514652f8 has broke GLOBAL_CLEANUP_PACKAGES by accidentally
excluding it from export (in favour of GLOBAL_CLEANUP_BASE_PACKAGES
that's been added then); fix that.

The problem at hand was that an installer component
of a "DVD class" image does use/cleanup/installer
while installable LiveCD component gets broken by that
(livecd-install -> installer-scripts-remount-stage2
which gets removed as installer-*).

Split those.

It appears that reusing installer-feature-*-stage3 packages
is perfectly fine with VM images; these just need to be removed
after the package scripts they carry have worked out.

It appears that features.in/Makefile functioned a bit
differently by now than was described back then: after
loops and pushds got rearranged for robustness, it stopped
to pick up a cleanup feature tagged script.

That particular script is now better de-tagged and simply
placed as a script to be merged into install2 subprofile.

The tagged scripts still require a bit more comprehension
to understand the use cases (e.g., do we need per-subprofile
tagged script subdirs or just a toplevel one should be looked
at, with script names telling where to put them).

README used to mix up subprofiles and features; fixed.