Thank you for your answer.
By inspecting catalyst examples specs, especially stage4 one, I figured that
catalyst could do the job.

Right now I proceed by "cross-emerging" from a Gentoo host to a target dir
with emerge's --root and --config-dir opts.
With that, I can start with an empty target dir, and emerge into it
baselayout, bash and glibc. That's all I need for now.
Then I cross compile needed kernel to my target dir following Gentoo
cross-dev guide :
http://www.gentoo.org/proj/en/base/embedded/cross-development.xml#doc_chap6

That's what I need. But that's all hand-crafted scripts to always build the
same things.
Plus, I will have to make my target dir a LiveCD just after, so I'll have to
script more.

I thought Catalyst could do the exact same thing with a more Gentoo-ish way
and sort-of cache settings.
But I'm still not sure. Plus I don't get the "stage"-related thing in spec.
I read FAQ and get it a little more.

Here are the 2 crucial points that I don't want to evade :
1) I do not want to build with a "deploy base Gentoo system, then remove"
strategy (won't this break things ?). I really want to start from nothing,
or from a almost-empty root dir/tar I define. Because I want my target
system to be a "oneshot" system. It will not use portage, or gcc. It will
just run applications.
2) I do not want my target system to bootstrap itselfs by compilating its
binaries. I want my host system to build target things. Because of 1)
reasons.

Can catalyst still do the job ?
Why exactly should I make a profile ?

2009/11/24 Peter Stuge <peter@stuge.se>

> Shinkan wrote:
> > - I want to build barely-usable minimal systems that I call
> > "guests" from my host. Guests would have to be setup on target
> > machines using fdisk and tar only, or put on a Live{CD,DVD,USB}.
> ..
> > Did I have to use Catalyst ? How can I do this if it's the clean
> > way ?
>
> You can certainly do it with a catalyst stage4 spec file. You'll also
> need to prepare a kernel configuration for the guests, and reference
> that in the spec file. You'll also spend some time on filling the
> spec file with packages, files and directories that should be
> unmerged and simply rm:ed from the final build.
>
> To make changes (like add another package to a guest) you would run
> catalyst again. It starts over, but keeps a cache of binpkgs that
> have been built so it runs well under an hour even on oldish systems.
>
> This is how I make custom distributions for customers.
>
> Since the guests will be very different from a standard Gentoo system
> it may also be worthwhile for you to create a new, custom, profile
> for the guests.
>
>
> //Peter
>
>


-- 
Pierre.
"Sometimes when I'm talking, my words can't keep up with my thoughts. I
wonder why we think faster than we speak. Probably so we can think twice." -
Bill Watterson