usb: drop HOST_USB

usb: drop HOST_USB

Nowdays we use libusb for usb-host, so we don't have different code
for linux vs. bsd any more.  So there is little reason to have the
HOST_USB variable, we can just write things directly into the Makefile
and avoid a pointless indirection.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 20170908111217.21985-2-kraxel@redhat.com