Fix blink1-tool for FreeBSD 10

* libusb_get_string_descriptor exists in FreeBSD 10
* FreeBSD, starting with FreeBSD 10, has its own iconv(3) implementation
* FreeBSD's iconv(3) is broken when linked statically

To build blink1-tool on FreeBSD 10 you have to run 'make FBSD10=yes'.