s390-ccw: use zipl values when no boot menu options are present

s390-ccw: use zipl values when no boot menu options are present

If no boot menu options are present, then flag the boot menu to
use the zipl options that were set in the zipl configuration file
(and stored on disk by zipl). These options are found at some
offset prior to the start of the zipl boot menu banner. The zipl
timeout value is limited to a 16-bit unsigned integer and stored
as seconds, so we take care to convert it to milliseconds in order
to conform to the rest of the boot menu functionality. This is
limited to CCW devices.

For reference, the zipl configuration file uses the following
fields in the menu section:

      prompt=1      enable the boot menu
      timeout=X     set the timeout to X seconds

To explicitly disregard any boot menu options, then menu=off or
<bootmenu enable='no' ... /> must be specified.

Signed-off-by: Collin L. Walling <walling@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>