Use per-device USB serial number strings.

Use per-device USB serial number strings.

These are not actual "serial numbers" in the strict sense, i.e. two
devices of the same type (e.g. two USBee ZX devices) would have the
same string.

Using two different actual serial numbers in such a case is not possible
since both devices would get the exact same firmware file (uploaded to
RAM via USB) and thus the exact same hardcoded "serial number". All devices
where fx2lafw is currently used don't have an EEPROM where the firmware
(and thus a different serial number) could be stored persistently; it's
always re-uploaded to the device's RAM upon connecting the device to USB.

However, the USB serial number field is still useful in that we can
store a nice device(-type) indicator string there. The benefit is that
an lsusb of an fx2lafw (after firmware upload) would yield something
specific like "USBee ZX" or "Hantek 6022BE" in addition to the very
generic "fx2lafw".