When using the wic image type, changes to wic variables don't cause do_rootfs to rerun. Looks like we need some explicit vardeps, since vars are exported and an external process is run, rather than being referenced directly.
Patch submitted to the list: http://lists.openembedded.org/pipermail/openembedded-core/2015-December/114704.html
Bug has been solved with commit: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=b1515064b5b4eb9b4c09c41436a559a907209860