sched: Remove task_delete in case of CONFIG_BUILD_KERNEL

Deleting a task from another task's context will not do, so shut
this gate down for BUILD_KERNEL. In this case if a task wants another
task to terminate, it must ask the other task to politely kill itself.

Note: kthreads still need this, also, the kernel can delete a task
without asking.

include/nuttx/kthread.h

@@ -154,20 +154,19 @@ int kthread_create(FAR const char *name, int priority, int stack_size,
  *   function is equivalent to task_delete.c; the following definition
  *   simply reserves the name in the name space.
  *
- *   Refer to comments with task_delete() for a more detailed description of
- *   the operation of this function.
+ *   Refer to comments with nxtask_delete() for a more detailed description
+ *   of the operation of this function.
  *
  * Input Parameters:
  *   pid - The task ID of the task to delete.  A pid of zero
  *         signifies the calling task.
  *
  * Returned Value:
- *   OK on success; or ERROR on failure with the errno variable set
- *   appropriately.
+ *   OK on success; or negated errno on failure.
  *
  ****************************************************************************/
 
-#define kthread_delete(p) task_delete(p)
+#define kthread_delete(p) nxtask_delete(p)
 
 #undef EXTERN
 #ifdef __cplusplus

include/sys/syscall_lookup.h

@@ -92,10 +92,11 @@ SYSCALL_LOOKUP(sem_wait,                   1)
 #ifndef CONFIG_BUILD_KERNEL
   SYSCALL_LOOKUP(task_create,              5)
   SYSCALL_LOOKUP(task_spawn,               6)
+  SYSCALL_LOOKUP(task_delete,              1)
 #else
   SYSCALL_LOOKUP(pgalloc,                  2)
 #endif
-SYSCALL_LOOKUP(task_delete,                1)
+
 SYSCALL_LOOKUP(task_restart,               1)
 SYSCALL_LOOKUP(task_setcancelstate,        2)
 SYSCALL_LOOKUP(up_assert,                  2)