hw/usb: Do not build USB subsystem if not required

If the Kconfig 'USB' value is not selected, it is pointless to
build the USB core components. Add a stub for the HMP commands
and usbdevice_create() which is called by usb_device_add in
softmmu/vl.c.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210424224110.3442424-3-f4bug@amsat.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 4c05ff8..6f7e5db 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1804,6 +1804,7 @@
 M: Gerd Hoffmann <kraxel@redhat.com>
 S: Maintained
 F: hw/usb/*
+F: stubs/usb-dev-stub.c
 F: tests/qtest/usb-*-test.c
 F: docs/usb2.txt
 F: docs/usb-storage.txt