Test: Build TF-M SPE alone with NS test flags selected

Some usage scenarios, such as multi-core platforms, require to build
non-secure side and secure side separately. TF-M currently supports to
build TF-M secure side with secure tests only.

However, some non-secure tests also require secure test services.
Therefore, NS test flags shall be selected even during TF-M secure build
alone.
But if NS tests flags are selected while building TF-M secure side only,
the TF-M build with current tf-m-tests will fail since the tf-m-tests is
not decoupled well.

This commit supports to build TF-M secure side only with NS test flags
selected, to include secure test services to support NS tests.
If NS is not selected, skip NS test framework and test suites build by
temporarily unsetting corresponding flags.

Signed-off-by: David Hu <david.hu@arm.com>
Change-Id: I2ae1caab9d5b63f43bb17453a3392cb22963f85d
8 files changed