Merge branch 'bugfix/separate_ethernet_and_wifi' into 'master'

Allow separate ethernet & wifi configuration

If only 1/2 of ethernet & WiFi are enabled in config, the other interface is no longer linked into the firmware.

* Fixes bug where enabling Ethernet but not WiFi would fail to compile.
* Also means that enabling WiFi but not Ethernet no longer links some unused ethernet interface functions.

See merge request !525