Created PPP service class that encapsulates the PPP protocol.
Class is similar to EMAC and L3IP classes with additional methods
to read IP and DNS server addresses negotiation using PPP and
to set PPP specific parameters (file handle for modem access etc.).
PPP service can use on its own thread or in run in mbed os event
Queue thread.
Added ppp_nsapi.cpp module that implements the nsapi_ppp.h
services.
Added ppp_nsapi.cpp module that implements the nsapi_ppp.h
services.
Mika Leppänen