debootstrap: fix use of sysvinit (Closes #992916)

Closes: #992916

I've tested the component debootstrap commands and the hack editing chroot/debootstrap/base during the two-stage setup. I have not though tested an actual build with live-build with the change due to lack of time. I don't expect it to be problematic, but I'm hoping/expecting that the user will try out my fix and confirm that it works.

(I'm afraid that I still do not have time to return to proper development of live build currently and do not know when I will be able to. I've just ended up throwing together this odd fix for an issue I happened to respond to on the ML, which actually took a lot longer than I hoped it would.)