Build: Improve handling of CMSIS RTX lib

Also add necessary platform files for the core testsuite

Change-Id: Ibfa4ba5f8c0beeca1f333f5be87c8ce8a0910ec9
Signed-off-by: Raef Coles <raef.coles@arm.com>
diff --git a/lib/ext/CMakeLists.txt b/lib/ext/CMakeLists.txt
index 25277d7..ced2557 100644
--- a/lib/ext/CMakeLists.txt
+++ b/lib/ext/CMakeLists.txt
@@ -10,6 +10,7 @@
 add_subdirectory(t_cose)
 add_subdirectory(mbedcrypto)
 add_subdirectory(tf-m-tests)
+add_subdirectory(CMSIS_5)
 if(BL2)
     add_subdirectory(mcuboot)
 endif()