toolchain/58536: build.sh should support cross-building packages into images

[campbell+netbsd%mumble.net@localhost]

>Number:         58536
>Category:       toolchain
>Synopsis:       build.sh should support cross-building packages into images
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    toolchain-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Aug 02 01:45:00 +0000 2024
>Originator:     Taylor R Campbell
>Release:        current, 10, 9, ...
>Organization:
The NetBSD Foundation.tgz
>Environment:
>Description:
It should be possible to do something like

./build.sh ... distribution package=net/socat release

and get socat binary package and/or release images that have it preinstalled.

Command-line argument might be better for saying `please build this package' than for saying `please include this package in the image' but it should be possible to say both of these things.  Maybe an mk.conf variable for the list of packages that should be included or something like that.
>How-To-Repeat:
seek to build cloud images or small appliance images that don't rely on the sordid state of TNF binary packages
>Fix:
Yes, please!

This will also require resolving an issue similar to https://gnats.netbsd.org/57639 which is that populating $DESTDIR/usr/pkg will make checkflist fail.  We'll need to find some way to resolve that.