nshlib: Split shell functionality from nsh to sh

1.Improve the compatiblity as other OS
2.Avoid call nsh_initialize more than once

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

system/nsh/sh_main.c

@@ -0,0 +1,53 @@
+/****************************************************************************
+ * system/nsh/sh_main.c
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+
+#include "nshlib/nshlib.h"
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: sh_main
+ ****************************************************************************/
+
+int main(int argc, FAR char *argv[])
+{
+  /* There are two modes that NSH can be executed in:
+   *
+   * 1) As a normal, interactive shell.  In this case, no arguments are
+   *    expected on the command line.  OR
+   * 2) As a single command processor.  In this case, the single command is
+   *    is provided in argv[1].
+   *
+   * NOTE:  The latter mode is only available if CONFIG_SYSTEM_NSH=m.
+   * In that case, this main() function will be built as a process.  The
+   * process will be started with a command by the implementations of the
+   * system() and popen() interfaces.
+   */
+
+  return nsh_system(argc, argv);
+}