These changes force to loadng instead of rpl unconditionally. We need to way to pick the modules in use from the application's makefile. I see two options:
- have a COMPILE_LOADNG makefile flag set before inclusion of the main Makefile.include. Also add a COMPILE_RPL, and unset it in this case
- or, move the module as an app, i.e., from `core/net/loadng` to `apps/loadng` and add APPS += loadng to the main makefile (and if you really need not to compile RPL we're screwed ;))

I prefer the first option