Instead of showing a warning, it is better to explicitly specify
the SYSLINUX_UI=none option in the config.

More than one BOOTLOADER may be available for the same architecture.
Set IMAGE_PACKTYPE in boot/iso, there is no point in duplicating
in different features.
Closes ALT bug 39786.

These are partial commits reverts:
01ee4075
e42bcdcb
5cfc7ede

...so that bits required for PXE boot are handy in the image.

Suggested-by: glebfm@altlinux.org, klark@altlinux.org

Every .iso was assumed to be bootable since the very beginning[*],
and isoboot images were deemed to be x86 isolinux ones; this didn't
change with basic ppc/armh support as I never ran into hardware
that would _boot_ those ISOs, not only run the code, and it was
only e2k isodata project that finally forced this refactoring.

It's still not perfect: pack and syslinux features still end up
somewhat interwoven, and too much places care for architecture
the image is being built for (instead of archdep features tossing
their appropriate bits and pieces in).

Should help:
- any-arch regarding isodata images;
- {x86,aarch64}/efi by decoupling isoboot and isolinux;
- ppc{,64} as introducing yaboot support will be easier now;
- mipsel{,64} too, hopefully.

* I knew of school addon images baked with mkimage-profiles-desktop
  but postponed and then neglected the whole problem for years...

This is more appropriate default value nowaday.

...instead of hardwired 0x314 (which is rather wrong
for modern kernels and hardware, just ran into that
with yet another Matrox G200eW WPCM450).

s/ALT Linux/ALT/g as per s/ALT Linux/BaseALT/g Ltd

Its immediate purpose was influencing the GRUB boot menu
*but* the implemented mechanism is actually a part of the
long planned text branding and might be further merged
into branding when hierarchical features finally chime in.

So let's get the naming straight before it breeds.

File contents should be correct.

As duly noted by glebfm@, branding issues need more attention
by now since only stage1/install2 got some of it so far in this
regard.  Hence the dedicated feature comes to the rescue
(well no, it doesn't actually mess with rescue!).

- incompatible change (to fix the rather broken early style):
  use/syslinux/ui-% is now use/syslinux/ui/%;

- default timeout changed to 9 seconds (long enough and keeps
  the countdown in a single figure);

- added totaltimeout of 300 seconds;

- provided live kiosk images with almost-instant boot by default;

...and some other assorted tweaks here and there, sorry.

As was noted by Alexey Shabalin in libosinfo context,
current ALT Linux images tend to lack ISO9660 metadata
-- which they did have back in the day of Master 2.4.

Please note that the data collection occurs this way
due to mkimage's config.mk resetting the values to be
empty; this was worked around by using another config
file, $(BUILDDIR)lib/iso.mk, and including it later
but that would require a separate target with per-target
CONFIG variable which isn't elegant at all given the need
to actually build up the metadata set.

So the variables were changed (to be more readable anyways)
and then proxied back to BOOT_*.  This might be cleaned up
some day after the inclusion order is tweaked or mkimage
defaults get set-if-unset-yet (?=).

The features might get copy-pasted (or even copied-and-pruned)
when initialized; there's an unneccessary duplication of the
function name in the line adding it to FEATURES list, thus
prone to being forgotten and causing some havoc later on.

It was wrong in the first place but tackling this with some
double-colon rules ran into terminality issues, and further
tortures were considered unneccessary.

The current solution isn't perfect (no completely transparent
function name registration upon corresponding target being called)
but at least it is an improvement...

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)

ToC:
- introduced doc/
- features.in/00example/
- READMEs and TODO for more
- code comments clarified

Some unrelated minor fixes bundled either.

Also included:
- reworked syslinux feature inclusion
- syslinux: s/textprompt/prompt/
- README++

- get rid of $dest
- work on features.in/syslinux
- README++