[sr-dev] [kamailio/kamailio] Commands of old makefiles with cmake (Issue #4052)

Opening this generic issue to track how some commands from old Makefiles can be done when using the new CMake-based build system, or what the possible alternatives. Hopefully we can build some knowledge here that can be then transferred to the wiki cmake tutorials.

### Compiling core only

With the old makefiles, the `make` compiled only the core of Kamailio. With cmake, `make` compiles everything.

Would it be possible to have a command with cmake that compiles only the core? Like:

``` make core ```

Or maybe there is a way to do it already?!?

### Compile a single module

With the old makefiles, compiling a single module, say acc, was possible with:

``` make modules modules=src/modules/acc ```

With cmake, this is possible with:

``` make acc ```