stm32/main: Guard usb_mode lines in default boot.py by relevant #if.

ports/stm32/main.c

@@ -136,8 +136,10 @@ static const char fresh_boot_py[] =
 "import machine\r\n"
 "import pyb\r\n"
 "#pyb.main('main.py') # main script to run after this one\r\n"
+#if MICROPY_HW_ENABLE_USB
 "#pyb.usb_mode('VCP+MSC') # act as a serial and a storage device\r\n"
 "#pyb.usb_mode('VCP+HID') # act as a serial device and a mouse\r\n"
+#endif
 ;
 
 static const char fresh_main_py[] =