Okay, after clearing caches, cleaning up env vars, etc. I'm able to reliably reproduce the problem.
I've attached two makefiles, one that works (in that, it builds an image, I don't care about running the image yet). The other fails to build with the aforementioned error. In the last version of live-build, I'm pretty sure the same command worked. The only difference between the two are the archive areas used; "firmware" should be a valid archive for Raspbian.
Incidentally, if anyone *has* a link to the old deb package for 1:20151215 so I could verify this, that'd help.