s390/ipl: support NVMe IPL kernel parameters

Enable extracting of extra kernel command-line parameters
from the NVMe IPL block passed by the firmware to the kernel
at boot.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>