Update nopersistent boot parameter

Originally created by @intrigeri on #10831 (Redmine)

Apparently, since live-boot (3.0~a27-1) and this commit:

commit 4e54d60d7f69ae3441e519e5611bd7ea48a8be19
Author: Daniel Baumann <daniel@debian.org>
Date:   Sun Apr 8 18:29:45 2012 +0200

    Using 'persistence' (noun) rather than 'persistent' (adjective/adverb) everywhere.

… we should use nopersistence instead of nopersistent on the kernel command line.

Strangely, db4e9230 updates the design doc while the code is unchanged. Too bad we didn’t have an automated test for the way live-boot handles persistence in the initramfs.

This may has security implications, to be confirmed (the fact we’re passing live-media=removable might help quite a bit).

Feature Branch: bugfix/persistence-fixes-for-2.0-take1