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
Added cellular modem libs to sync script
Added missing constructor for CDMA modem initializer
Added CellularModem as an abstract base class for modem implementations

Now both UbloxUSBGSMModem and UbloxUSBCDMAModem inherit from
CellularModem.
Updated tests

- Vodafone tests are gone for now
- Added u-blox TCP test (UB_1) and SMS test (UB_2).
- tests can now have more than a single source directory