CMake: Remove utility function dependency on mbed-os target

The utility function may be called by targets other than mbed-os
when it is broken into multiple libraries.

cmake/util.cmake

@@ -4,8 +4,7 @@
 # PREFIX - TARGET or similar (can be empty string)
 # KEYWORD_LIST - list of labels that are used to include directories
 function(mbed_add_cmake_directory_if_labels PREFIX)
-    get_target_property(target_labels mbed-os MBED_TARGET_LABELS)
-    foreach(key ${target_labels})
+    foreach(key ${MBED_TARGET_LABELS})
         if(NOT "${PREFIX}" STREQUAL "")
             string(PREPEND key ${PREFIX} "_")
         endif()