[S390] vmcp: disallow modular build

Change the tristate Kbuild option into a bool option so that the module
is either builtin or not available at all.
There have been too many cases where people were missing the 'vmcp'
device node and unable to send z/VM CP commands. So let's make sure
that on distros it will always be present.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>