Remove circular dependency on WiFi/LWIP_Ethernet (#1415)

Fixes #1408

The wl_* types are required in multiple libraries which in turn seem
to have a circular dependency that Platform.IO has issues with in certain
modes.

Avoid the issue by moving the common headers into the core directories
so they will be accessible by all libs.

Move StackThunk to core directory, too

Author: earlephilhower