NSH start up

1.nsh init and start up

1. apps/examples/nsh/nsh_main.c

   up_cxxinitialize(); // initialization of the static C++ class instances
   
   /* Initialize the NSH library */
   nsh_initialize();
   {
      /* Mount the /etc filesystem */
      (void)nsh_romfsetc();
   }
   nsh_consolemain(0, NULL);