fix(psa): align with original API in tf-m-extras

The measured boot API is available in the tf-m-extras
repo:
partitions/measured_boot/interface/src/measured_boot_api.c

This change make the API behavior align with
the original implementation.

Signed-off-by: Tamas Ban <tamas.ban@arm.com>
Change-Id: Ie4af38b859f942b2ef090e92da64d75811b5b49b