sched_releasetcb() will normally free the stack allocated for a task. However, a task with a custom, user-managed stack may be created using nxtask_init() followed by nxtask_activer. If such a custom stack is used then it must not be free in this many or a crash will most likely result.
This chagne addes a flag call TCB_FLAG_CUSTOM_STACK that may be passed in the the pre-allocted TCB to nxtask_init(). This flag is not used internally anywhere in the OS except that if set, it will prevent sched_releasetcb() from freeing that custom stack.