Optionally remove OS dependencies

[Sympatron]

In an effort to make it easier to port LibOSDP to embedded devices, I introduced the NO_OS flag (name is up for debate).
If this flag is set, no calls to system functions like printf, puts or gettimeofday will be made.

Instead the user is expected to set a either a custom logger->puts function or a logging callback.
For time keeping the user is expected to provide a usecs_now function, which is way easier and more useful on embedded devices.

Asserts are redirected to the global logger instead of printf.

Fixes #27

[Sympatron]

An alternative approach would be automatically detect the absence of an OS if neither the Linux nor the Windows defines are set.

I am very open for debate if you don't like this specific implementation.

[sidcha]

@Sympatron As always, thanks for the PR. I need to take a closer look at this problem before I can respond here. Will do so as soon as I get some spare cycles :).

[sidcha]

Okay, just had a look at this. The way I see it, when LibOSDP is built for bare metal targets, the app should provide a strong reference to a osdp_millis_now which would solve the gettimeofday issue. Perhaps I'm missing something?

For puts and printf, my rationale was to provide a reasonable default. Many embedded SDKs have putchar redirection means into a uart device which would allow those functions to either log to uart or become NOPs. But if they cause issues, we can remove the defaults and expect callers to set something explicitly.

Edit: Okay, I think I see what you mean: libosdp itself doesn't need it but the compilation units force the need to build utils.c which has this problem. Let me think more about this.

[Sympatron]

Closed in favor of #29.