improve USB host library and cellular modem stack

This pull request contain improvements for the USB host library and cellular modem library
- modified the USBHostSerial to support devices with more than one CDC port
- allow to configure the debug level of the USB stack
- added serial port support to cellular modem (required for u-blox SARA-G modems)
- update cellularmodem library for better separation
- update of UbloxUSBModem library to support above changes, removed the power control feature as this is handeled external by the C027 library