make usb devices configurable

Leave the core usb devices (usb hub, tablet, mouse, keyboard)
enabled unconditionally.  Make the other ones configurable.

Exceptions:
  - bluetooth: not qdevified yet, has a vl.c dependency because
    of that, thus disabling isn't as easy as not linking the
    object file.
  - smardcard: ccid-card-emulated depends on that one *and*
    CONFIG_SMARTCARD_NSS.  So it isn't a one-liner and comes
    as separate patch because of that.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/default-configs/x86_64-softmmu.mak b/default-configs/x86_64-softmmu.mak
index 233a856..3392f5a 100644
--- a/default-configs/x86_64-softmmu.mak
+++ b/default-configs/x86_64-softmmu.mak
@@ -1,6 +1,7 @@
 # Default configuration for x86_64-softmmu
 
 include pci.mak
+include usb.mak
 CONFIG_VGA=y
 CONFIG_VGA_PCI=y
 CONFIG_VGA_ISA=y