add switch to support disabling USBSerial

Added switch ("DISABLE_SERIALUSB") to allow a user to disable the USBSerial port completely.

I've successfully implemented a [usbMassStorage](https://github.com/joeferner/maple-usbMassStorage) device driver by porting the STMicro code for the maple but to work with the Maple USB library I needed to use User_Standard_Requests, Device_Property, and Device_Table global variables. In order to do this I need to disable the existing cdcacm driver.