do not delete SEC_UPDATES info from inithooks.conf

currently we delete the inithooks.conf before we process
the security updates scripts. This patch keeps the SEC_UPDATES
parameter and makes it possible to override the setting
in /etc/default/inithooks with the setting in the custom
/etc/inithooks.conf file.

Signed-off-by: Peter Lieven <pl@kamp.de>
Merge pull request #3 from plieven/sec_updates

do not delete SEC_UPDATES info from inithooks.conf