Initial import of USB3GModem in the USBHost library

In the future, USBHostConf.h should really reside in the project that
imports USBHost, not inside the USBHost library itself. Doing that now
though might break compatibility with projects that currently import
USBHost, so we need to figure out a better solution.
Refactored dongle initialization code

The code for dongle initialization (WANDongleInitializer.cpp) was removed from
this class, it will be added by each modem as needed.
Removed references to specific initializers from the code
Renamed USB3GModule to USBHost3GModule to keep the naming convention
Moved USBSerialStream from the CellularModem library to USBHost

This removes any dependency on USB code from CellularModem.
Remove "debugging LED" from USBHost.cpp
Changed USBHostConf.h to better match the previous USBHost configuration.