kernel/os: Add helper CONTAINER_OF macro

[andrzej-kaczmarek]

This is a convenience macro to calculate original pointer of "parent"
object using a pointer to one of its members. It's commonly used in
e.g. Linux kernel and Zephyr Project so it should be also useful in
Mynewt :)

For example, with following structure:

struct foo {
    int a;
    struct bar bar;
    int c;
}

Let's assume we have:

struct foo g_foo;
struct bar *x = &g_foo.bar;

And we can use CONTAINER_OF to calculate pointer to g_foo using x:

struct foo *foo = CONTAINER_OF(x, struct foo, bar);

[mkiiskila]

For this particular macro, I'd prefer container_of in small caps. To match offsetof(). But I'm not hung up on it.

[andrzej-kaczmarek]

I also considered it as a lowercase for the same reason and also this is how it's defined in Linux kernel - but since it's still not full consistent with offsetof (i.e. without underscore) I eventually did this as in Zephyr which has the advantage that we port some code from there so there will be less changes required :-)