Initial Posix PSE52 Support

[nashif]

Provide a POSIX compatibility Layer on top of native APIs:

From the list below, implement whatever is applicable with Zephyr kernel features:

• 1003.1b-93 Realtime Signals
• 1003.1b-93 Semaphores
• 1003.1b-93 Hi Res. Clocks & Timers
• 1003.1b-93 Shared Memory
• 1003.1b-93 IPC Message Passing
• 1003.1c-95 Threads
• 1003.1c-95 Thread Safe Functions
• 1003.1c-95 Thread Attribute Stack Address
• 1003.1c-95 Thread Attribute Stack Size
• 1003.1c-95 Thread Priority Scheduling
• 1003.1c-95 Thread Priority Inheritance
• 1003.1c-95 Thread Priority Protection

[LorenzMeier]

@nashif Could you share the roadmap for this or a planning document?

[nashif]

@LorenzMeier first set of patches to support Posix was merged, there are still APIs that will follow in upcoming releases, for example filesystem.