This makes libusb handling more consistent: if any driver requires
libusb, first check for libusb-1 availability, if not found and
libusb0 is ok for this case, try libusb0.
A user-visible bug fixed by this is that when a user was selecting a
driver that required libusb-1 but was lacking the necessary dev files
or libraries, configure didn't complain (but the build was failing of
--enable-stlink doesn't really require libusb-1 but since it's using the
same automake symbol HLAADAPTER, ti_icdi is getting built too which needs
libusb1 unconditionally. Since using libusb-0.1 makes little sense anyway,
leave that as is.
Signed-off-by: Paul Fertser <firstname.lastname@example.org>
Reviewed-by: Spencer Oliver <email@example.com>