Most of the time it's fine to run with parallelism enabled (particularly PARALLEL_MAKE), however, if you're trying to debug an issue with the actual makefiles/build scripts you often need to disable it. We ought to provide an option to "devtool build" to turn it off.
bitbake -r (or -R ? ) would let you specify a file for overriding env vars. As suggested by Paul.
Set of patches sent the OE mailing list.
Patch merged in master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=0d0e50810a793f517b253e329339ba40d4bedd36
Verified on master: fc45deac89ef63ca1c44e763c38ced7dfd72cbe1