Now that the old drivers have been converted to libusb1, there is
no need to keep the build dependency from libusb0.

Drop libusb0 from configure and makefiles, remove the libusb0
helper and remove libusb0 and libusb-compat from the README files.

Change-Id: Icc05be74ae5971ba6cbb67d39107c709a4d826e6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5993
Tested-by: jenkins