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.

install2 cleanups:

- functionally indifferent ones: particularly, install2/*/98system's
  "mkdir -p /image" was superfluous as it was done by that time already
  by sub.in/stage2/image-scripts.d/00stage1

- taken apart, prepared for tags: so far it's a mostly moot change
  since the installer cleanup scripts themselves are mostly the same as
  preceding 90cleanup was (with some additions corresponding to recent
  kernel development); it's still unclear what the mechanism for
  configuring the cleanups in effect will be, either directory/package
  regex lists or tagged scripts excluded from execution by yet another tag

fixes:

- image.in/Makefile: fix metadata related test; the actual test was
  assuming that stage1 kernel means installer, which is not the case
  since generic stage2 introduction; oh well

- 85cleanup-lowmem: a "_" too much was the culprit in destroying the
  needed translations along with those deemed superfluous; thanks go to
  Oleg Ivanov and Lenar Shakirov for finding the bug and proposing the
  fix altogether

additions:

- features.in/Makefile: reworked help target; it was rather inaccessible
  due to BUILDDIR normally undefined at the time of direct make
  invocation, and BUILDDIR is normally defined during normal builds
  anyways so let's try it this way.

- README++

daydreams:

- 01-genbasedir: we should drop bzip2 compressed pkglists some day
  but see genbasedir and apt-cdrom first, 90-pkg.sh (alterator-pkg)
  will fail miserably otherwise

This is a sort of anti-feature which removes and not builds;
still with mkimage-profiles' approach we can at least build
up the removal procedures as well.

It's what triggered the tagged scripts, BTW.