configs: specify tf-a build options for tf-a-tests test build

Specify the TF-A build options to be used when building the test for
tf-a-tests. This overrides the default TF-A build options specified in
the config files. This change applies to RD-N2, RD-N2-Cfg1 and RD-V2
platforms.

Change-Id: I016218c61e5c1fba598d0a43224e0e2bee8edaa5
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>