s390/boot: Add cmdline option to relocate lowcore

Now that everything has been converted, add the option
'relocate_lowcore' to enable relocating the lowcore.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>