Ethernet interface

Adds an additional Ethernet RPC interface to pxarCore.

The additional feature is disabled by default. The USB connection should work as before, just (if compiled with that feature) the second interface will be checked for DTBs, too.

There are changes to the build behaviour:
* BUILD_dummydtb is removed
* INTERFACE_USB and INTERFACE_ETH are added
* To build the dummy DTB class, just set both interface flags to OFF
* Ethernet is default OFF, USB is default ON, both can be built at the same time, too

Standing issues (to be addressed in time when DTBs with Ethernet interface are available)
* Currently the interface to be used (`eth0`) is hardcoded. This should be made a config parameter and be provided to pxarCore when creating a new instance.