sys: net: Initial import of a general interface to a network protocol
sys: net: Initial import of a dummy MAC protocol layer
sys: transceiver: Provide an adapter to basic_net module

Since basic_net is aimed to replace tranceiver one day, but porting
efforts usally take a while, this commit aims to provide access via
transceiver as we are used to for radio/network drivers that already implement
basic_net (e. g. uart_net in #1454).
basic_net: dummy_mac: Add capability to fire events with net_dev_dummy
unittests: provide unittests dummy_mac