trying (again) to clarify pkg_postinst[_ontarget]

["Robert P. J. Day" <rpjday@crashcourse.ca>]

  i know i once asked about this (probably on oe-core list), but i was
reading this section in the dev manual,

http://docs.yoctoproject.org/dev-manual/common-tasks.html#post-installation-scripts

and i think it needs work, or i just have no idea how to read it
properly, so let me ask some really dumb, basic questions.

  first, IIRC, once upon a time, such a script had the option of being
run at image install time, or on-target install time (depending on i
think whether the script exited with 0 or 1 or something like that).
those days are gone, and the new design is to use one of:

  pkg_postinst
  pkg_postinst_ontarget

fair enough, but in the old days, such a script might check the value
of ${D} to see if it was running in the image context, or on-target
context, and obviously run differently. given the current setup with
those two distinct routines, is there any purpose in such a script
*ever* checking the value of ${D} anymore? that is, is it safe to
assume that the first version above is run in image context, and the
second in on-target context? or is there still some obscure reason to
need to check?

  i ask as i see this in base-passwd_3.5.29.bb:

    pkg_postinst:${PN}-update () {
    #!/bin/sh
    if [ -n "$D" ]; then
            exit 0
    fi
    ${sbindir}/update-passwd
    }

  how does one interpret that? or is just a holdover from the old
days? certainly that section in the docs is not going to help a reader
understand the (still) numerous references to ${D} in some of those
scripts.

rday