configure: remove --enable-libusb0 option, rework libusb selection

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
course).

--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 <fercerpav@gmail.com>
Change-Id: I630a4ad9e4583743d45b2233bf5d8f4e5c0aab82
Reviewed-on: http://openocd.zylin.com/1434
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/Makefile.am

@@ -19,18 +19,15 @@ SUBDIRS=
 # Standard Driver: common files
 DRIVERFILES += driver.c
 
-if USB
-DRIVERFILES += usb_common.c
-endif
-
 if USE_LIBUSB1
 DRIVERFILES += libusb1_common.c
-else
-
-if USE_LIBUSB0
-DRIVERFILES += libusb0_common.c
 endif
 
+if USE_LIBUSB0
+DRIVERFILES += usb_common.c
+if !USE_LIBUSB1
+DRIVERFILES += libusb0_common.c
+endif
 endif
 
 if BITBANG