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.

Some images (i.e. docker and systemd-bare) require apt
but do not require etcnet.

lib/*.mk aren't going to be parsed for build targets
in the near future; and the early placement of those
targets was superseded by a dedicated configuration
snippet directory so just move these bits there.

This one is finally there and should help reduce the
systemd-induced mess in package lists and specifications.

Actually the templates pretending to be usable missed the whole
interactivesystem (sysvinit would get pulled in by services as well).

Fixed somewhat but time and practice will tell.

There's no need to repeat the typical openssh-* triade
all over the place; those who need server and client
are better off pulling in "openssh" pkglist, and those
needing a particular package should specify it.

openssh-server is in need indeed on almost any server instance;
thanks Aleksey Cheusov for reporting the shortage.

This might be amended in the future but is reasonable right now.

A minimal chroot supporting extension via apt-get;
vitals if built on Sisyphus as of Jan 16, 2012:

  i586: 13M tar.xz, 58M chroot (33M w/o /usr/share/{doc,locale,man})
x86_64: 14M tar.xz, 60M chroot (35M w/o /usr/share/{doc,locale,man})

Trivial fixups (extra checks) added to two script hooks.

This was asked for by Leo-sp50 and torabora, and seems quite reasonable:
let's provide means to keep at least some distribution configurations
a bit apart, so that these can be considered more standalone in terms
of hard warranted functionality but at the same time enjoying the common
infrastructure.

Considering lib/distro.mk: it's now experimentally pulled apart so that
parallel development of different distro families can go on without
major merge hassles.  *Please* don't abuse with massive copy-paste.

And before you ask: this might get extended to allow for "private"
out-of-tree configurations being included since apparently there
are goals with no meaning outside of some very particular context...
but otherwise I'd like to encourage getting reusable bits in-tree.

This is quite a large-scale change since mkimage-profiles got used to
baking distributions over the last year, and virtual environments are
quite different, so e.g. image.in/Makefile had to be split in two with
the main part of it moved into features.in/iso/lib/.

Short overview:

- features.in/Makefile: lib/ support
  (supporting VE images requires dynamic modifications
  to image.in/Makefile before starting the build;
  the most natural way to achieve that seems to use
  features mechanism along with makefile include dir)

- packaging format related part moved into features.in/pack
  (should be better prepared for diversity either)
- features.in/iso renamed to features.in/build-distro
- features.in/ve  renamed to features.in/build-ve
  + NB: these could not be merged as e.g. features.in/build
    due to completely different script hooks

- lib/image.mk renamed to lib/build.mk
- image, config, log postprocessing moved downstream
- added a sort of a topping in the form of lib/sugar.mk
- assorted style fixups (like ifeq usage)

- clean.mk: reliability fix (the problem was observed by Oleg Ivanov
  and me too but finally it did get the attention quantum)

- reviewed, updated and extended docs
  + QUICKSTART: should be[come] a step-by-step guide
    (thanks Leo-sp50 for prodiving feedback)