build_helper_configs.py: Select target tfm_ns when BL2=OFF

tfm_app_binaries are built only when BL2=ON.
Build target tfm_ns instead when BL2=OFF.
Pass PLATFORM_DEFAULT_IMAGE_SIGNING to NS build to control image signing
together with BL2.

It is a temporal workaround to mitigate the difference between
toolchains.

Change-Id: I221d16fd36ddbd5a655bb0132c94ebefcd344b8c
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/tfm_ci_pylib/tfm_build_manager.py b/tfm_ci_pylib/tfm_build_manager.py
index 7721a8c..091e855 100644
--- a/tfm_ci_pylib/tfm_build_manager.py
+++ b/tfm_ci_pylib/tfm_build_manager.py
@@ -87,6 +87,15 @@
 
         return toolchain
 
+    def choose_ns_target(self, with_bl2):
+        ns_target = ""
+        if with_bl2:
+            ns_target = "tfm_app_binaries"
+        else:
+            ns_target = "tfm_ns"
+
+        return ns_target
+
     def get_compiler_name(self, compiler):
         compiler_name = ""
         if "GCC"in compiler:
@@ -388,6 +397,7 @@
                             "tfm_platform": i.tfm_platform,
                             "s_compiler": self.choose_toolchain(i.compiler, s_build = True),
                             "ns_compiler": self.choose_toolchain(i.compiler, s_build = False),
+                            "ns_target": self.choose_ns_target(i.with_bl2),
                             "isolation_level": i.isolation_level,
                             "test_regression": self.map_params(i.test_regression, mapRegTest),
                             "test_psa_api": i.test_psa_api,