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).
[WIP] Add tests for dummy_mac (+ basic_net)
[WIP] Initial import of uart_net header definition