s390/kconfig: split the s390 base menu

Split the base menu into more descriptive categories like processor,
memory, etc. and move virtualization related entries to the
Virtualization menu.
Also select KEXEC unconditionally.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>