[yocto] Out-of-tree module build against yocto kernel

[Ed]

I want to build an out-of-tree kernel module against my Yocto-generated
kernel (cross-compiled, not on the target).
I know that I can modify the hello-mod recipe to build it alongside the
Yocto image, but I'd strongly prefer not to; it's not a workflow that's
conducive to rapid iteration during development & test.

I've tried building against the kernel in
<TMP>/work-shared/<MACHINE>/kernel-source, to no avail. While I don't
understand everytihng that's going on, I suspect it has something to do
with how Yocto organizes kernel build artifacts (autoconf headers,
Module.symvers, conf stuff).

I've also tried building a SDK. By default that didn't seem to include
kernel headers, so I added kernel-devsrc to TOOLCHAIN_TARGET_TASK in
populate_sdk_base.bbclass. That seemed to get me kernel source w/ the SDK,
but I still haven't been able to successfully build against it.

A great deal of my troubleshooting has been driven by the a 2013 discussion
on this list that appeared to end inconclusively:
https://lists.yoctoproject.org/pipermail/yocto/2013-January/011564.html

Am I going about this in the right way? Am I just missing some
configuration/setup steps, or am I going about this all wrong? Everything
I'm trying feels clunky and hack-ish.

-- 
Ed Laird
On two occasions I have been asked,—"Pray, Mr. Babbage, if you put into the
machine wrong figures, will the right answers come out?" ... I am not able
rightly to apprehend the kind of confusion of ideas that could provoke such
a question.
—Charles Babbage, Passages from the Life of a Philosopher
<http://en.wikipedia.org/wiki/Charles_Babbage>
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