fix ML reported issue

 - fix the evaluation of kiwi initrd behavior
 + the <oem-kiwi-initrd> element accepts true|false values. These settings
    are propagated without changes to the shell code that evaluates the
    variable to decide whether or not to create a new initrd. The evaluation
    in the shell code was based on yes|no and old relic that used to be
    use in the XML a long time ago. This update changes the evaluation in
    the shell code to true|false.