Required mkimage 0.3.0-alt1

The commit 99695713 used this variable
even though it didn't exist yet. A good reason to add this variable.

This is a fix to previous failures of
ve/vm + use/repo/main build attempts
(in fact, any non-distro/ targets).

SUBDIRS were just optimized away...

BASE_PACKAGES_REGEXP and THE_PACKAGES_REGEXP,
to be exact; the lack of handling these appears
to have been the culprit of firefox missing in
vm images which use/browser/firefox.

Classic VEs don't carry any kernel since these are running
under a single OpenVZ (or potentially LXC) kernel image;
ARM Multiboot (TWRP in this particular case) allows to boot
off a chroot via kexec, and we need a kernel in it for that,
obviously.

No bootloader required inside such VE though.

There was some extra duplication, just clean it up.

This further refines the modular build by making
metadata being a clearly separated feature rather
than having to rely on runtime tests, and also by
moving the code which cares for kernel bits of base
installation (.base list) in a feature of its own.

There's more to it but let's get the ball rolling first.

Actually this is the proper rewrite that was looming ever since
tgz support was introduced: there are multiple archive formats
supported by mkimage, and there are multiple compression methods
available as well.

So the bullet got bitten yet again along with the "goal parser"
which should be more straightforward by now.

Thanks dkr@ and mithraen@ for the inspiration of this evening.

It is a current convention to prefer clearly phony targets
(see the wiki page) so let's follow it here too.

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)