netutils/cmux: Add support for CMUX protocol

This commit adds CMUX (GSM 07.10) protocol support to netutils.
CMUX allows multiplexing multiple virtual serial connections
over a single physical serial link.

Changes include:
- CMUX protocol implementation
- CRC table for frame validation
- Basic frame handling

Signed-off-by: Halysson <halysson1007@gmail.com>

include/netutils/cmux.h

@@ -0,0 +1,66 @@
+/****************************************************************************
+ * apps/include/netutils/cmux.h
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * 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.
+ *
+ ****************************************************************************/
+
+#ifndef __APPS_INCLUDE_NETUTILS_CMUX_H
+#define __APPS_INCLUDE_NETUTILS_CMUX_H
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <sys/time.h>
+#include <stdbool.h>
+#include <debug.h>
+#include <errno.h>
+
+/****************************************************************************
+ * Public Types
+ ****************************************************************************/
+
+struct cmux_settings_s
+{
+    FAR const char *tty_name;
+    FAR const char *script;
+    int total_channels;
+};
+
+/****************************************************************************
+ * Public Function Prototypes
+ ****************************************************************************/
+
+#undef EXTERN
+#if defined(__cplusplus)
+#define EXTERN extern "C"
+extern "C"
+{
+#else
+#define EXTERN extern
+#endif
+
+int cmux_create(struct cmux_settings_s *settings);
+
+#undef EXTERN
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* __APPS_INCLUDE_NETUTILS_CMUX_H */