Build: Align change tf-m-tests config pass

tf-m-tests might pass in empty config file path or directory. Update the
parse of those config value to align with tf-m-tests change.

Change-Id: I8bf0c0a62002ac806f4e9cb9f19766ffeebe0d9b
Signed-off-by: David Hu <david.hu@arm.com>
2 files changed