usb/storage: add kconfig symbols

Add new kconfig symbols so usb-storage and usb-bot can
be enabled or disabled individually at build time.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20210312090425.772900-5-kraxel@redhat.com>
diff --git a/hw/usb/Kconfig b/hw/usb/Kconfig
index 40093d7..53f8283 100644
--- a/hw/usb/Kconfig
+++ b/hw/usb/Kconfig
@@ -66,11 +66,22 @@
     default y
     depends on USB
 
+config USB_STORAGE_CORE
+    bool
+    depends on USB
+    select SCSI
+
+config USB_STORAGE_CLASSIC
+    bool
+    default y
+    depends on USB
+    select USB_STORAGE_CORE
+
 config USB_STORAGE_BOT
     bool
     default y
     depends on USB
-    select SCSI
+    select USB_STORAGE_CORE
 
 config USB_STORAGE_UAS
     bool