s390/ipl: only print boot menu error if -boot menu=on was specified

s390/ipl: only print boot menu error if -boot menu=on was specified

It is possible that certain QEMU configurations may not
create an IPLB (such as when -kernel is provided). In
this case, a misleading error message will be printed
stating that the "boot menu is not supported for this
device type".

To amend this, only print this message iff boot menu=on
was provided on the commandline. Otherwise, return silently.

While we're at it, remove trailing periods from error
messages.

Signed-off-by: Collin L. Walling <[email protected]>
Message-Id: <1519760121[email protected]>
Reviewed-by: Thomas Huth <[email protected]>
Acked-by: Christian Borntraeger <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>