kernel: add context pointer to thread->fn_abort

For compatibility layers like CMSIS where thread objects
are drawn from a pool, provide a context pointer to the
exited thread object so it may be freed.

This is somewhat obscure and has no supporting APIs or
overview documentation and should be considered a private
kernel feature. Applications should really be using
k_thread_join() instead.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

include/kernel.h

@@ -286,8 +286,26 @@ struct k_thread {
 
 	/**
 	 * abort function
-	 * */
-	void (*fn_abort)(void);
+	 *
+	 * This function pointer, if non-NULL, will be run once after the
+	 * thread has completely exited. It may run in the context of:
+	 *   - the idle thread if the thread self-exited
+	 *   - another thread calling k_thread_abort()
+	 *   - a fatal exception handler on a special stack
+	 *
+	 * It will never run in the context of the thread itself.
+	 *
+	 * A pointer to the thread object that was aborted is provided. At the
+	 * time this runs, this thread object has completely exited. It may
+	 * be re-used with k_thread_create() or return it to a heap or slab
+	 * pool.
+	 *
+	 * This function does not run with any kind of lock active and
+	 * there is the possibility of races leading to undefined behavior
+	 * if other threads are attempting to free or recycle this object
+	 * concurrently.
+	 */
+	void (*fn_abort)(struct k_thread *aborted);
 
 #if defined(CONFIG_THREAD_MONITOR)
 	/** thread entry and parameters description */