nsh/nshlib: Add alias support for nsh

This adds support for string aliases into nsh. There are some nuances that
are not handled correctly yet:

- Reserved words can be overloaded, which is a clear POSIX violation

nshlib/nsh.h

@@ -715,6 +715,25 @@ struct nsh_parser_s
 #endif
 };
 
+#ifdef CONFIG_NSH_ALIAS
+struct nsh_alias_s
+{
+  FAR struct nsh_alias_s *next;    /* Single link list for traversing */
+  FAR char               *name;    /* Name of the alias */
+  FAR char               *value;   /* Value behind the name */
+  union
+  {
+    struct
+    {
+      uint8_t             exp : 1; /* Already expanded ? */
+      uint8_t             rem : 1; /* Marked for deletion */
+    };
+
+    uint8_t               flags;   /* Raw value */
+  };
+};
+#endif
+
 /* This is the general form of a command handler */
 
 struct nsh_vtbl_s; /* Defined in nsh_console.h */
@@ -793,6 +812,13 @@ int nsh_usbconsole(void);
 #  define nsh_usbconsole() (-ENOSYS)
 #endif
 
+#ifdef CONFIG_NSH_ALIAS
+FAR struct nsh_alias_s *nsh_aliasfind(FAR struct nsh_vtbl_s *vtbl,
+                                      FAR const char *token);
+void nsh_aliasfree(FAR struct nsh_vtbl_s *vtbl,
+                   FAR struct nsh_alias_s *alias);
+#endif
+
 #ifndef CONFIG_NSH_DISABLESCRIPT
 int nsh_script(FAR struct nsh_vtbl_s *vtbl, FAR const char *cmd,
                FAR const char *path, bool log);
@@ -1193,6 +1219,11 @@ int cmd_pmconfig(FAR struct nsh_vtbl_s *vtbl, int argc, FAR char **argv);
 #  endif
 #endif
 
+#ifdef CONFIG_NSH_ALIAS
+int cmd_alias(FAR struct nsh_vtbl_s *vtbl, int argc, FAR char **argv);
+int cmd_unalias(FAR struct nsh_vtbl_s *vtbl, int argc, FAR char **argv);
+#endif
+
 /****************************************************************************
  * Name: nsh_extmatch_count
  *