We introduced a method on the BitbakeShellCommand base class for unifying the way in which a buildstep is skipped, we should adopt this pattern throughout the codebase but shouldn't do so until after M1.
There are relatively few spots left where adopting using this method makes sense, but a patch has been written to do so.
skipStep() used everywhere it makes sense, most recently: http://git.yoctoproject.org/clean/cgit.cgi/yocto-autobuilder/commit/?id=f70c7bf903ab62a7b2db540b2d33881166c383bd