nshlib: Make nsh_telnetmain() in public

Summary:
- This commit makes nsh_telnetmain() in public so that we can
  call it from applications.
- Also, CONFIG_NSH_DISABLE_TELNETSTART is introduced so that
  we can disable nsh_telnetstart()

Impact:
- nsh with telnet daemon

Testing:
- Tested with sabre-6quad:netknsh (will be updated later)

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

include/nshlib/nshlib.h

@@ -126,6 +126,27 @@ void nsh_initialize(void);
 
 int nsh_consolemain(int argc, FAR char *argv[]);
 
+/****************************************************************************
+ * Name: nsh_telnetmain
+ *
+ * Description:
+ *   This interface may be called or started with task_start to start a
+ *   single NSH instance that operates on stdin and stdout for telnet daemon.
+ *   This function does not return.
+ *
+ * Input Parameters:
+ *   Standard task start-up arguments.  These are not used.  argc may be
+ *   zero and argv may be NULL.
+ *
+ * Returned Values:
+ *   This function does not normally return.  exit() is usually called to
+ *   terminate the NSH session.  This function will return in the event of
+ *   an error.  In that case, a non-zero value is returned (EXIT_FAILURE=1).
+
+ ****************************************************************************/
+
+int nsh_telnetmain(int argc, FAR char *argv[]);
+
 /****************************************************************************
  * Name: nsh_telnetstart
  *