OpenOCD Support: Re-introduce uxTopUsedPriority (#188)

* re-introduce uxTopUsedPriority. Prevent removal by optimization

* Make uxTopUsedPriority volatile to avoid optimizer + code comment

Co-authored-by: Cobus van Eeden <35851496+cobusve@users.noreply.github.com>

tasks.c

@@ -379,6 +379,11 @@ PRIVILEGED_DATA static UBaseType_t uxTaskNumber = ( UBaseType_t ) 0U;
 PRIVILEGED_DATA static volatile TickType_t xNextTaskUnblockTime = ( TickType_t ) 0U; /* Initialised to portMAX_DELAY before the scheduler starts. */
 PRIVILEGED_DATA static TaskHandle_t xIdleTaskHandle = NULL;                          /*< Holds the handle of the idle task.  The idle task is created automatically when the scheduler is started. */
 
+/* Improve support for OpenOCD. The kernel tracks Ready tasks via priority lists.
+ * For tracking the state of remote threads, OpenOCD uses uxTopUsedPriority
+ * to determine the number of priority lists to read back from the remote target. */
+const volatile UBaseType_t uxTopUsedPriority = configMAX_PRIORITIES - 1U;
+
 /* Context switches are held pending while the scheduler is suspended.  Also,
  * interrupts must not manipulate the xStateListItem of a TCB, or any of the
  * lists the xStateListItem can be referenced from, if the scheduler is suspended.
@@ -2093,6 +2098,10 @@ void vTaskStartScheduler( void )
     /* Prevent compiler warnings if INCLUDE_xTaskGetIdleTaskHandle is set to 0,
      * meaning xIdleTaskHandle is not used anywhere else. */
     ( void ) xIdleTaskHandle;
+    
+    /* OpenOCD makes use of uxTopUsedPriority for thread debugging. Prevent uxTopUsedPriority 
+     * from getting optimized out as it is no longer used by the kernel. */
+    ( void ) uxTopUsedPriority;
 }
 /*-----------------------------------------------------------*/