Also changed usbdev, usbsn, usbvendor and usbproduct components from
PROGRAMMER structure to be cached string pointers rather than fixed-size
arrays. These will be initialised by pgm_new() with a pointer to nul;
Since commit ed46f09c1ccd1351e003a200ba50e3e4778ac478 (Implement
tc[io]flush methods & deprecate broken purge_buffers methods.)
ftdi_usb_purge_buffers() routine is deprecated. Use HAVE_FTDI_TCIOFLUSH
macro to invoke the newly introduced ftdi_tcioflush() routine.