split f_setup_securix_system function

into:
- f_setup_securix_system
- f_setup_genkernel
- f_setup_portage

Because `f_setup_securix_system` is only interesting for securix users, while `f_setup_genkernel` and `f_setup_portage` are more general functions, interesting for general gentoo hardened interested ones (such as for Whonix Gentoo Hardened Port).