some questions about pkg_postinst* scripts

  in an attempt to document *all* of the features of post-installation
scripts (and prerm and so on), i've been reading the docs, and asking
google, and looking at various examples in the OE code base, so i have
the first of some questions so i can improve that subsection (which, i
submit, really deserves to be a full section of the dev manual on its
own, and not just a small part of the "new recipe" section). so ...

  first, if a recipe defines a "pkg_postinst:${PN}" script (or some
related pkg_postinst:package script), that script will *always* be
invoked on the build host when that package is added to the image
during do_rootfs(), is that correct? ignore whether the script is not
meant to run on the build host, or whether it checks ${D}, or whether
it exits with a non-zero exit code ... leave all that to the side, is
it true that it will always at least be *invoked*, regardless of its
contents? what it does after being invoked we will leave until later.

  related to that, the only possibility for it to not be invoked on
the build host at all is to define it as
pkg_postinst_ontarget:package(), yes?

  let's start there.

rday