config: use radio_address_t
config: moved from core to sys

There's no need to run config_load from core directly, can be done by
auto_init.
config: make members feature specific

Channel and address are only sensible for boards that actually have a
transceiver.
config: suppress cppcheck warnings
Merge pull request #1900 from OlegHahm/config_radio_address_t

config: use radio_address_t in config_t