s390/boot: access kernel command line via parmarea

Access the kernel command line via parmarea instead of using the
COMMAND_LINE define.
With this the following gcc11 warning is removed:

arch/s390/boot/ipl_parm.c: In function ‘setup_boot_command_line’:
arch/s390/boot/ipl_parm.c:168:50: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]

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