Feature: Mutex with Priority Inheritance Protocol

[iankuan]

In current patch, we already implemented a futex-liked mutex. It didn't support PI. In order to achieve better real-time capability, the higher priority task should execute as soon as possible. It also makes sense on synchronization. So we could apply PI to enhance our real-time capability.

By the way, this's feature would be released soon. Furthermore, folks should not work on this feature directly, because we already have a preliminary idea and some progress on it.

[1] LK Documentation - rt-mutex.txt Linux Kernel Documentation
[2] LK Documentation - rt-mutex-design.txt Linux Kernel Documentation
[3] LK Documentation - PI-futex.txt
[3] ChibiOS - RT Mutexes and Condition Variables
[4] Nuttx - Signaling Semaphores and Priority Inheritance
[5] x15.git