Remove USBSerialStream from USBHost

The code from USBSerialStream depends on some headers from CellularModem,
so remove it from USBHost.
Changed debugging macros in WANDongle and WANDongleSerialPort

Now they use "dbg.h" which is a part of USBHost, thus eliminating the
dependency on code from CellularModem.
Modifications for compatibility with GCC
Added CellularModem library, removed old VodafoneUSBModem library