[RTOS] Added idle hook API

[neilt6]

Added a new API for attaching an idle hook to be executed by the idle task. This will allow developers to add custom logic to the idle task (e.g. to call sleep()) without modifying the library code.

[neilt6]

Here's an example program demonstrating the new API:

#include "mbed.h"
#include "rtos.h"

DigitalOut led1(LED1);

void idleHook()
{
    sleep();
}

int main()
{
    Thread::attach_idle_hook(idleHook);

    while(1) {
        led1 = 1;
        Thread::wait(500);
        led1 = 0;
        Thread::wait(500);
    }
}

[sg-]

Did you consider using the FunctionPointer class for handling the user handler?

[neilt6]

I wasn't sure I could access a C++ class instance from os_idle_demon(), and it made sense to keep the RTX layer pure C.

[neilt6]

Is this pull request still being considered, or should I withdraw it?

[0xc0170]

@neilt6 We will have a second look at this, will let you know the upcoming week.

cc @sg-

[adamgreen]

This functionality looks like a good addition. I do have a few comments/questions:

• There are some files you touched where you used tabs for indentation when the original code used spaces. It would be good to keep things consistent and use spaces.
• Why did you place rtos_idle_hook_fptr in the global namespace and not just have a C function that application code could call to attach/detach to a module scoped function pointer (ie. static)? The NULL pointer check you currently have in the C++ attach/detach method could then be pushed down into that C function.

[neilt6]

Thanks for the feedback @sg- and @adamgreen. I'm going to withdraw this for now, and see if there's a better way to implement this functionality.