s390/kexec: do not automatically select KEXEC option

Following commit dccf78d3 ("kernel/Kconfig.kexec: drop
select of KEXEC for CRASH_DUMP") also drop automatic KEXEC
selection for s390 while set CONFIG_KEXEC=y explicitly for
defconfig and debug_defconfig targets. zfcpdump_defconfig
target gets CONFIG_KEXEC unset as result, which is right
and consistent with CONFIG_KEXEC_FILE besides.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>