Minor - compile out usb-msc code if CONFIG_USB_MSC not set.
diff --git a/src/usb-msc.c b/src/usb-msc.c
index e91d916..52fb58a 100644
--- a/src/usb-msc.c
+++ b/src/usb-msc.c
@@ -111,6 +111,8 @@
 int
 process_usb_op(struct disk_op_s *op)
 {
+    if (!CONFIG_USB_MSC)
+        return 0;
     switch (op->command) {
     case CMD_READ:
         return cdb_read(op);