Build: Unify isolation level setting with TFM_ISOLATION_LEVEL

Configuration to set isolation level is unified with TFM_ISOLATION_LEVEL
in TF-M build system. Same change is also needed in tf-m-tests to keep
the codes aligned.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I12abc8e01bf7e52d49f09809549d551c2244b540
1 file changed