Build: Skip building platform_ns when NS app is not selected

Set EXCLUDE_FROM_ALL property in platform_ns target to skip
building platform_ns target when NS app is disabled.

Change-Id: I2e9c5122ec3253d510806d5cdf3d18ee3e4b1985
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/interface/CMakeLists.txt b/interface/CMakeLists.txt
index 3af7daa..37ecfbc 100644
--- a/interface/CMakeLists.txt
+++ b/interface/CMakeLists.txt
@@ -15,7 +15,7 @@
     tfm_toolchain_reload_compiler()
     # The platform target is created in this directory/file so that it has the
     # same settings as the main ns target.
-    add_library(platform_ns STATIC)
+    add_library(platform_ns STATIC EXCLUDE_FROM_ALL)
 endif()
 
 ###################### PSA interface (header only) #############################
@@ -51,17 +51,6 @@
         $<$<BOOL:${FORWARD_PROT_MSG}>:FORWARD_PROT_MSG=${FORWARD_PROT_MSG}>
 )
 
-# A dummy interface library to make sure platform NS build can proceed when NS
-# is disabled, if a platform doesn't use NS flag to control platform NS build.
-if (NOT NS)
-    add_library(tfm_ns_interface INTERFACE)
-
-    target_link_libraries(tfm_ns_interface
-        INTERFACE
-            psa_interface
-    )
-endif()
-
 ###################### PSA api (S lib) #########################################
 
 target_sources(tfm_secure_api