bitbake-setup configurations for existing LTS releases

Hello,

I've been asked to look into adding bitbake-setup configurations for
existing LTS releases (e.g. scarthgap).

TLDR: this is not impossible, but bitbake-setup relies on a few
important features in oe-core and bitbake, and so such configurations
would be limited, and awkward and not really showcase bitbake-setup
well - basically reducing it to an equivalent of "clone poky;
oe-init-built-env", with more steps and hurdles.

Specifically:

- any bitbake environment produced will not have bitbake-setup in it
(as opposed to master and future yocto releases). So one would have to
continue to refer to bitbake-setup from a separate bitbake master
checkout, instead of just using one in PATH.

- fragment support is available only in walnascar. Built-in fragment
support (for setting machine and distro!) is only available in master.
Bitbake-setup relies on both, or it can use openembedded templates as
a fallback.

- openembedded templates have issues too: we have only one default
template in poky (which assumes integrated poky repo and can't be used
with separate oe/bitbake/meta-yocto clones), and one default template
in openembedded.

- the poky template hardcodes qemux86-64 for MACHINE and poky for
DISTRO. Openembedded template similarly hardcodes qemux86-64. It's not
possible to select something else, compared to built-in fragments that
make it trivial.

- for best first-time experience it's also good to enable yocto sstate
mirror. In master, this is done with a fragment. In scarthgap, this
would have to be enabled some other way, and I really can't think of
which way that would be.

So given the above, I'm leaning towards not trying to retrofit
bitbake-setup to the past. But what do you think?

Alex