Allowed overriding of cdcacm strings and IDs

You can now define CDCACM_MAN, CDCACM_PROD and CDCACM_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:
-DCDCACM_MAN="Digilent" -DCDCACM_PROD="chipKIT UNO32"

I have also made it so that CDCACM_VID, CDCACM_PID and CDCACM_RID can be overridden with defines too.