shell: Add TELNET backend

Add TELNET backed for shell module. The TELNET implementation is based
on the telnet_console driver.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

include/shell/shell_telnet.h

@@ -0,0 +1,75 @@
+/*
+ * Copyright (c) 2019 Nordic Semiconductor ASA
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef SHELL_TELNET_H__
+#define SHELL_TELNET_H__
+
+#include <shell/shell.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+extern const struct shell_transport_api shell_telnet_transport_api;
+
+/** Line buffer structure. */
+struct shell_telnet_line_buf {
+	/** Line buffer. */
+	char buf[CONFIG_SHELL_TELNET_LINE_BUF_SIZE];
+
+	/** Current line length. */
+	u16_t len;
+};
+
+/** TELNET-based shell transport. */
+struct shell_telnet {
+	/** Handler function registered by shell. */
+	shell_transport_handler_t shell_handler;
+
+	/** Context registered by shell. */
+	void *shell_context;
+
+	/** Buffer for outgoing line. */
+	struct shell_telnet_line_buf line_out;
+
+	/** Network context of TELNET client. */
+	struct net_context *client_ctx;
+
+	/** RX packet FIFO. */
+	struct k_fifo rx_fifo;
+
+	/** The timer is used to send non-lf terminated output that has
+	 *  been around for "too long". This will prove to be useful
+	 *  to send the shell prompt for instance.
+	 */
+	struct k_timer send_timer;
+
+	/** If set, no output is sent to the TELNET client. */
+	bool output_lock;
+};
+
+#define SHELL_TELNET_DEFINE(_name)					\
+	static struct shell_telnet _name##_shell_telnet;		\
+	struct shell_transport _name = {				\
+		.api = &shell_telnet_transport_api,			\
+		.ctx = (struct shell_telnet *)&_name##_shell_telnet	\
+	}
+
+/**
+ * @brief This function provides pointer to shell telnet backend instance.
+ *
+ * Function returns pointer to the shell telnet instance. This instance can be
+ * next used with shell_execute_cmd function in order to test commands behavior.
+ *
+ * @returns Pointer to the shell instance.
+ */
+const struct shell *shell_backend_telnet_get_ptr(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* SHELL_TELNET_H__ */