sched/Kconfig: waitpid() depends on SCHED_HAVE_PARENT if BUILD_KERNEL

waitpid() cannot be used in kernel mode unless SCHED_HAVE_PARENT is
selected -> add dependency if BUILD_KERNEL is selected.

Why? Because without SCHED_HAVE_PARENT waitpid() works in a non-standard
way, meaning it does not use SIGCHLD to wake the parent, as it should.

Also, returning the child status via stat_loc corrupts memory as stat_loc
points to the parent's address environment:

pid_t nxsched_waitpid(pid_t pid, int *stat_loc, int options)
{
  ...
  group->tg_statloc = stat_loc;
  ...
}

And later when the status is returned, the child writes to tg_statloc,
which points to the parent's address environment -> random memory
corruption:

static inline void nxtask_exitwakeup(FAR struct tcb_s *tcb, int status)
{
  ...
  if (group->tg_statloc != NULL)
    {
      *group->tg_statloc = status << 8;
    }
  ...
}

sched/Kconfig

@@ -755,13 +755,15 @@ endif # SCHED_CHILD_STATUS
 config SCHED_WAITPID
 	bool "Enable waitpid() API"
 	default n
+	depends on SCHED_HAVE_PARENT || !BUILD_KERNEL
 	---help---
 		Enables the waitpid() interface in a default, non-standard mode
 		(non-standard in the sense that the waited for PID need not be child
 		of the caller).  If SCHED_HAVE_PARENT is also defined, then this
 		setting will modify the behavior or waitpid() (making more spec
 		compliant) and will enable the waitid() and wait() interfaces as
-		well.
+		well. Note that SCHED_HAVE_PARENT must be defined in BUILD_KERNEL if
+		SCHED_WAITPID is needed.
 
 config SCHED_DUMP_LEAK
 	bool "Enable catch task memory leak"