check conditions of  make  for equality of variables with an empty value
instead of check definition.
A defined but empty variable under all these conditions results errors.

The comment was incomplete since commit
1f13b3a0.

Failing to kaboom just in time can make (and has just made)
it harder to figure out the culprit down the road; e.g.
metadata target's tar(1) might fail to find *some* files
that haven't been copied over due to a single one missing
by that run.

This is to support Centaurus-like images featuring
more than a single installation profile (e.g.,
"workstation/server/minimal").

Quite a few more modifications are needed to do it
right though as m-p has fallen prey to exactly the
"single root composition" assumption with its current
THE_* variables semantics and processing.

- better *_LISTS printout
- somewhat cleaner .base comments
- more comprehensible git log messages
  + special handling for complex subprofiles
  + no need to commit build.log ;-)

Thanks go to lakostis@ (see #18047) for all the aic94xx reminders
and the final testing of the proposed image.

- image.in/functions.mk: rework kpackage()
  + it takes two arguments explicitly now: this adds some noise
    for "generic" invocations but is rather less messy with recently
    introduced STAGE1_KFLAVOUR (which in its turn is rather cleaner
    than messing with KFLAVOURS, especially since soemthing changed
    in presumably apt and we can't rely on kernel packages being
    installed in the order formed).
- BUILDDIR/DEBUG related fixes
  + Makefile: BUILDDIR initialization moved to distro.mk
- build.log += git info

- more thorough build stages inline doc
- quieter BUILDDIR instantiation
- branding-altlinux-sisyphus by default
- prettify installer selection somewhat
- mkimage #24669 fixed upstream
- sub.in/stage1/modules: +drm (as in m-p-d, for KMS)
- tags2lists: generalize a bit

...along with obituary fixes.

- syslinux: menu.c32 works
- features: initial draft
  + memtest
- mv pkg/ pkg.in/
  + server-base: include openssh-blacklist by default
- mv isodata/ copy/
- README: changed to utf8, added an URL

These feel quite interesting but not that obvious yet...

Building m-p-d stuff in-tree was plagued by accidental
generated file commits, and it wasn't nice at all anyways.

As not every package list is useful for every distro,
only the needed lists should be copied for a particular one.