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.

Author: ianstcdns

include/mpu_wrappers.h

@@ -288,9 +288,17 @@
 
 #else /* portUSING_MPU_WRAPPERS */
 
-    #define PRIVILEGED_FUNCTION
-    #define PRIVILEGED_DATA
-    #define FREERTOS_SYSTEM_CALL
+    #ifndef PRIVILEGED_FUNCTION
+        #define PRIVILEGED_FUNCTION
+    #endif
+
+    #ifndef PRIVILEGED_DATA
+        #define PRIVILEGED_DATA
+    #endif
+
+    #ifndef FREERTOS_SYSTEM_CALL
+        #define FREERTOS_SYSTEM_CALL
+    #endif
 
 #endif /* portUSING_MPU_WRAPPERS */