selftests/kexec: Add missing '=y' to config options

so the file can be used as kernel config snippet.

Signed-off-by: Petr Vorel <pvorel@suse.cz>
[zohar@linux.ibm.com: remove CONFIG_KEXEC_VERIFY_SIG from config]
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>