Initial work by Bartek Szatkowski in https://github.com/ARMmbed/mbed-os/pull/4079,
reworked following review of https://github.com/ARMmbed/mbed-os/pull/5202 to
transform the entire system into C++, retaining the basic functionality.

Bartek's summary:

* Porting ethernet to EMAC
* Updating EMAC to enable multiple interfaces
* Untangling networking classes, making the abstractions a bit clearer to follow, etc
* General refactoring
* Removal of DEVICE_EMAC flag and introducing DEVICE_ETH and DEVICE_WIFI

Revisions since initial branch:

* Remove lwip depencies
* Correct doxygen warnings
* Remove emac_api.h, replace with C++ EMAC abstract class.
* Create OnboardNetworkInterface, and LWIP implementation.
* Mappings since #4079
     lwip-interface/nsapi_stack_lwip.c -> LWIPStack.cpp
     lwip-interface/ipstack_lwip.c -> LWIPInterface.cpp
     netsocket/mbed_ipstack.h -> OnboardNetworkStack.h
     hal/emac_api.h -> EMAC.h
* Reinstate use of EthInterface abstraction
* Correct and clarify HW address EMAC ops
* Restore MBED_MAC_ADDR implementation
* Integrate PPP support with LWIP::Interface.
* Convert K64F lwIP driver to K64F_EMAC.

To do:

* Convert emac_stack_mem.h to follow this pattern.
* Figure out DEVICE_ETH/EMAC
* Update all drivers to use EMAC