Compiling and linking against target libraries when using SDK/ImageBuilder

[Shankar Unni]

I'm trying to distribute an SDK and an ImageBuilder from an OpenWrt
build, so that another team can compile their code (in a different
repo), link against target libraries (e.g. openssl, etc.), and then
create an image including their packages on top of the stock packages.

Way back (a few years ago) when we did this last, this all worked -
the SDK contained target includes and libraries, and you could just
use -L and -I options to pick them up when compiling outside the
openwrt build tree.   (The idea here is to spare that team the need to
build all of openwrt for each build of their code - that takes a
considerable amount of time.)

Now, however, I see that all the target stuff has been dropped from
the SDK (since 2016!), but then it's not included anywhere else either
(ImageBuilder, or even Toolchain).

How can I distribute the prebuilt SDK and ImageBuilder in such a way
that they can compile and link against target packages?

Thanks in advance!