debianutils: Convert to update-alternatives.bbclass

This causes the configuration to be done at build time
rather than on first boot.  It also fixes errors in the
postinstall scripts that were not getting caught.

Submitted for upstream review and merge here:
    http://lists.openembedded.org/pipermail/openembedded-devel/2014-June/096576.html
    http://lists.openembedded.org/pipermail/openembedded-devel/2014-June/096577.html

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>