cmake: zephyr: Implement used function

Implements add_subdirectory_ifdef function needed
to build with TF-M.

Signed-off-by: Flavio Ceolin <flavio.ceolin@gmail.com>

Author: Fla-Pinto

CMSIS/CMakeLists.txt

@@ -1,3 +1,9 @@
 # SPDX-License-Identifier: Apache-2.0
 
+function(add_subdirectory_ifdef feature_toggle source_dir)
+  if(${${feature_toggle}})
+    add_subdirectory(${source_dir} ${ARGN})
+  endif()
+endfunction()
+
 add_subdirectory_ifdef(CONFIG_HAS_CMSIS_CORE_M Core)