jtag/libusb_helper: permit adapters to compute their custom serials

jtag/libusb_helper: permit adapters to compute their custom serials

introduce a callback function that could be passed to jtag_libusb_open
to permit adapters to compute their custom/exotic serials.

the callback should return a non NULL value only when the serial could not
be retrieved by the standard 'libusb_get_string_descriptor_ascii'

Change-Id: Ib95d6bdfc4ee655eda538fba8becfa183c3b0059
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5496
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>