[TF-M] Latest status of S/NS build split PoC

Hi all,

I rebase and update the S/NS build split PoC as requests. Could you please take a look at the latest patch set if you are interested? Trusted-firmware-m part: https://review.trustedfirmware.org/q/project:TF-M%252Ftrusted-firmware-m+bra... Tf-m-tests part: https://review.trustedfirmware.org/q/project:TF-M%252Ftf-m-tests+branch:mast...

The general idea was presented in https://www.trustedfirmware.org/docs/tf-m_forum_20220120_Restructure_TF-M_bu... while some details are improved in code.

Any comment and feedback is welcome!

Best regards, Hu Ziji