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