supervisor.mk: Conditionally assign USB_DEVICES

This restores the ability to remove CDC and/or MSC, at the price of
giving up the new automatic check that USB_DEVICES is correct.
Since devices have to have CDC and MSC to be "CircuitPython",
this is not a facility that is going to be used by any in-tree drivers.

Author: jepler

supervisor/supervisor.mk

@@ -122,7 +122,7 @@ endif
 ifeq ($(CIRCUITPY_USB_HID),1)
 USB_DEVICES_COMPUTED := $(USB_DEVICES_COMPUTED),HID
 endif
-USB_DEVICES := "$(USB_DEVICES_COMPUTED)"
+USB_DEVICES ?= "$(USB_DEVICES_COMPUTED)"
 
 ifndef USB_HID_DEVICES
 USB_HID_DEVICES = "KEYBOARD,MOUSE,CONSUMER,GAMEPAD"