s390/boot: do not change default_lma

The current modification of the default_lma is illogical and should be
avoided. It would be more appropriate to introduce and utilize a new
variable vmlinux_lma instead, so that default_lma remains unchanged and
at its original "default" value of 0x100000.

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