s390/arch: install kernels with their proper version ID

In case $INSTALLKERNEL is not available, we should install the kernel
image with its version number, and save the previous one accordingly.
Also, we're adding a hint so users know that they still need to
perform one more configuration step (usually adjusting zipl config).

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