Better RTOS support with mutex callbacks

I wanted to use TF with FreeRTOS and run into an issue with concurrent accesses. To solve this issue, TF version 1.2 contains two callback functions to claim and release a mutex when composing/sending a frame.

For an example see the new TF_Integration.example.c file.

This release also (finally) provides a TF_VERSION constant that can be used to log the used TF version and also know what version you have installed.

Please avoid editing the library code and instead discussing required new features here in the issue tracker so they can be added. The library files should be immutable and easy to update.