selftests/kexec: make kexec_load test independent of IMA being enabled

Verify IMA is enabled before failing tests or emitting irrelevant
messages.

Suggested-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Dave Young <dyoung@redhat.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>