SerialEthernetInterface.cpp is compiled for every nRF52 target because
PlatformIO builds all .cpp files under src/. It includes
SerialEthernetInterface.h, which unconditionally pulls in
<RAK13800_W5100S.h> -- a library only present in the RAK4631 Ethernet
env's lib_deps. As a result any other nRF52 board (e.g. Heltec T114)
fails to build with 'RAK13800_W5100S.h: No such file or directory'.
Wrap the contents of both files in '#ifdef ETHERNET_ENABLED' so they
compile to empty translation units on non-Ethernet builds. RAK4631
Ethernet envs define ETHERNET_ENABLED and are unaffected.
Fixes#2985
Ryan Gregg
cc221330ba