kernel: add RT_DEBUG_IN_THREAD_CONTEXT

In thread context means: 1) the scheduler has been started; 2) not in
interrupt context. It is more stronger than RT_DEBUG_NOT_IN_INTERRUPT.
With this commit, you will catch the error on situations like taking
mutex before scheduling instead of crashing on NULL pointer reference.
Merge pull request #170 from grissiom/rtt-next

kernel: add RT_DEBUG_IN_THREAD_CONTEXT