nrf/main: Call usb_cdc_init() before executing boot.py and main.py.

Otherwise, there is no USB available when running main.py, and main.py
cannot be interrupted with Ctrl-C.

ports/nrf/main.c

@@ -257,16 +257,16 @@ soft_reset:
 
     led_state(1, 0);
 
+    #if MICROPY_HW_USB_CDC
+    usb_cdc_init();
+    #endif
+
     #if MICROPY_VFS || MICROPY_MBFS || MICROPY_MODULE_FROZEN
     // run boot.py and main.py if they exist.
     pyexec_file_if_exists("boot.py");
     pyexec_file_if_exists("main.py");
     #endif
 
-    #if MICROPY_HW_USB_CDC
-    usb_cdc_init();
-    #endif
-
     for (;;) {
         if (pyexec_mode_kind == PYEXEC_MODE_RAW_REPL) {
             if (pyexec_raw_repl() != 0) {