selftests/kexec: check kexec_load and kexec_file_load are enabled

Skip the kexec_load and kexec_file_load tests, if they aren't configured
in the kernel.  This change adds a new requirement that ikconfig is
configured in the kexec_load test.

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