Add the possibility to directly configure the desktop app
for livecd kiosk mode to start.

This should have been done as soon as livecd-runapp
hit the streets -- livecd-{0ad,fgfs} definitely must go
as adding a line into config is trivially done near
the only place that requires them (conf.d/live.mk).

TODO: might be more generally useful for vm/* too,
      take this out into a separate feature then.

Edited: https://lists.altlinux.org/pipermail/devel-distro/2022-September/002889.html

It's been an old intent to add/move feature-specific
variable documentation into the features themselves,
this might be a pilot project. :-)

Hardly belonged there in the first place and became a culprit
during armh branch development since it had to be forked in
an ugly manner; move to rootfs hooks and be done with it.

VM images will be able to benefit either *but* installed systems
might have some trouble when this is implemented:
http://lists.altlinux.org/pipermail/devel/2013-May/197447.html

Split off use/live/x11 as a common free/proprietary ground either
(this refactoring had to be performed in parallel with x11 feature
being revamped, diffs quickly became intertangled unfortunately).

There are three more variables to adjust the redirect page:
HOMEPAGE, HOMENAME and HOMEWAIT; e.g. live-webkiosk is now
quite a bit more whitelabel.

- introduced generic stage2 subprofile (non-standalone)
- ported installer and rescue over to stage2/{install2,rescue}
- initial stage2/live (needs more work for sure)
- use make-initrd-propagator
- updated and somewhat extended doc/

NB: mind #26133, #26134