Make ETHERNET configuration the default if DEVICE_EMAC is present,
instead of if FEATURE_LWIP is present.
This limits it to targets which have been ported to the new EMAC API.
Add LWIP feature to JSON config, as in principle the targets shouldn't
be adding it themselves. Opens scope to having Nanostack-based tests.
Disable tests for the Realtek and Wifi drivers that aren't ported yet.
Tools will figure out if user passed in a custom path (to indicate configration file for module) or whether they used a keyword (to indicate they'd like to use an mbed OS configuration)