Fix nasa#2311, correct fallback file case

Only include the "FALLBACK_FILE" if the normal search came up empty.
Do not return a list containing the fallback/default along with the
user-supplied files, only return the user-supplied files.  This was an
issue when using "ALLOW_LIST" in that it had both.

cmake/global_functions.cmake

@@ -46,14 +46,6 @@ function(cfe_locate_implementation_file OUTPUT_VAR FILE_NAME)
 
   # Build the list of possible locations for this file in REVERSE priority order
   set(IMPL_SEARCH_PATH)
-  if (LOCATEIMPL_ARG_FALLBACK_FILE)
-    if (IS_ABSOLUTE "${LOCATEIMPL_ARG_FALLBACK_FILE}")
-      list(APPEND IMPL_SEARCH_PATH "${LOCATEIMPL_ARG_FALLBACK_FILE}")
-    else()
-      list(APPEND IMPL_SEARCH_PATH "${CMAKE_CURRENT_LIST_DIR}/${LOCATEIMPL_ARG_FALLBACK_FILE}")
-    endif()
-  endif()
-
   # Check for the existence of the file in each of the dirs
   foreach(BASEDIR ${IMPL_SEARCH_BASEDIRS})
     list(APPEND IMPL_SEARCH_PATH "${BASEDIR}${FILE_NAME}")
@@ -74,6 +66,14 @@ function(cfe_locate_implementation_file OUTPUT_VAR FILE_NAME)
     endif()
   endforeach()
 
+  if (NOT SELECTED_FILE AND LOCATEIMPL_ARG_FALLBACK_FILE)
+    if (IS_ABSOLUTE "${LOCATEIMPL_ARG_FALLBACK_FILE}")
+      set(SELECTED_FILE "${LOCATEIMPL_ARG_FALLBACK_FILE}")
+    else()
+      set(SELECTED_FILE "${CMAKE_CURRENT_LIST_DIR}/${LOCATEIMPL_ARG_FALLBACK_FILE}")
+    endif()
+  endif()
+
   if (SELECTED_FILE)
     message(STATUS "Using file: ${SELECTED_FILE} for ${FILE_NAME}")
   elseif(NOT LOCATEIMPL_ARG_OPTIONAL)