Optional PRIVILEGED_DATA wrapper (FreeRTOS#1433)

- Instead, when MPU wrappers are not used, allow port to override
  PRIVILEGED_FUNCTION, PRIVILEGED_DATA, or FREERTOS_SYSTEM_CALL
  selectively, permitting custom section placement at link-time.
- Allows critical data to be placed in port-specific location for
  improved performance, notably for cache-coherent SMP.
- Does not require enabling the full MPU wrappers.

Cherry pick + merge required to maintain submodule pointers.

Author: ianstcdns

include/mpu_wrappers.h

@@ -280,17 +280,19 @@
     #define PRIVILEGED_DATA         __attribute__( ( section( "privileged_data" ) ) )
     #define FREERTOS_SYSTEM_CALL    __attribute__( ( section( "freertos_system_calls" ) ) )
 
-#elif ( defined portMOVE_PRIVILEGED_DATA )
+#else /* portUSING_MPU_WRAPPERS */
 
-    #define PRIVILEGED_FUNCTION
-    #define PRIVILEGED_DATA         portMOVE_PRIVILEGED_DATA
-    #define FREERTOS_SYSTEM_CALL
+    #ifndef PRIVILEGED_FUNCTION
+        #define PRIVILEGED_FUNCTION
+    #endif
 
-#else /* portUSING_MPU_WRAPPERS */
+    #ifndef PRIVILEGED_DATA
+        #define PRIVILEGED_DATA
+    #endif
 
-    #define PRIVILEGED_FUNCTION
-    #define PRIVILEGED_DATA
-    #define FREERTOS_SYSTEM_CALL
+    #ifndef FREERTOS_SYSTEM_CALL
+        #define FREERTOS_SYSTEM_CALL
+    #endif
 
 #endif /* portUSING_MPU_WRAPPERS */