s390/kexec: Make KEXEC_SIG available when CONFIG_MODULES=n

s390/kexec: Make KEXEC_SIG available when CONFIG_MODULES=n

The commit c8424e776b09 ("MODSIGN: Export module signature definitions")
replaced the dependency of KEXEC_SIG on SYSTEM_DATA_VERIFICATION with the
dependency on MODULE_SIG_FORMAT. This change disables KEXEC_SIG in s390
kernels built with MODULES=n if nothing else selects MODULE_SIG_FORMAT.

Furthermore, the signature verification in s390 kexec does not require
MODULE_SIG_FORMAT because it requires only the struct module_signature and,
therefore, does not depend on code in kernel/module_signature.c.

But making ARCH_SUPPORTS_KEXEC_SIG depend on SYSTEM_DATA_VERIFICATION is
also incorrect because it makes KEXEC_SIG available on s390 only if some
other arbitrary option (for instance a file system or device driver)
selects it directly or indirectly.

To properly make KEXEC_SIG available for s390 kernels built with MODULES=y
as well as MODULES=n _and_ also not depend on arbitrary options selecting
SYSTEM_DATA_VERIFICATION, set ARCH_SUPPORTS_KEXEC_SIG=y for s390 and select
SYSTEM_DATA_VERIFICATION when KEXEC_SIG=y.

Fixes: c8424e776b09 ("MODSIGN: Export module signature definitions")
Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>