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/build_helper/build_helper_configs.py b/build_helper/build_helper_configs.py
index 5b63ce6..835e031 100755
--- a/build_helper/build_helper_configs.py
+++ b/build_helper/build_helper_configs.py
@@ -74,7 +74,7 @@
 
     # CMake build commands will be executed for every build.
     "spe_cmake_build":  "cmake --build %(ci_build_root_dir)s/spe -- install",
-    "nspe_cmake_build": "cmake --build %(ci_build_root_dir)s/nspe -- tfm_app_binaries",
+    "nspe_cmake_build": "cmake --build %(ci_build_root_dir)s/nspe -- %(ns_target)s",
 
     "set_compiler_path": "export PATH=$PATH:$%(compiler)s_PATH",