Update 'nopersistent' boot parameter to 'nopersistence'.

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 have been passing 'nopersistence' instead of 'nopersistent' on the
kernel command line.

Will-fix: #10831