Allowed overriding of cdcacm strings and IDs

You can now define USBCDC_MAN, USBCDC_PROD and USBCDC_SER using -D in the board's flags (or wherever else is suitable) with quoted strings and the USB CDC will then use those as string descriptors.  Great for identifying specific boards with udev.

eg:
-DUSBCDC_MAN="Digilent" -DUSBCDC_PROD="chipKIT UNO32"

I have also made it so that CDCACM_VID, CDCACM_PID and CDCACM_RID can be overridden with defines too.

I have just noticed that I chose different names (USBCDC / CDCACM) for my symbols - maybe I should change it?