[TF-M] Out-of-tree secure partition build support

Hi all,

Can I ask you to review the following patch set to enable out-of-tree secure partition build in TF-M? https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/10562/

The purpose is to enable developers to develop their own secure partitions outside TF-M repo. Developers can maintain their own code and repos, independently. Developers can pass their out-of-tree secure partition paths via TF-M command line, to build out-of-tree partitions with TF-M together.

For more details, please check the updated document: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/10696 Suggestions and comments are welcome!

Best regards, Hu Ziji