shell: add select command

The "select" command has been implemented, which allows user to
narrow down the command tree.

This implementation differs from the "select" command available
in the legacy shell. In a new implementation, if the selected
command has a handler and if the user has not entered the
registered subcommand, the shell will call the handler of selected
command and pass the text as arguments.

This may be useful, for example, if someone wants to use the
shell as an interface to a modem that supports AT commands.
Instead of each time you write e.g:
	at at+command1
	at at+command2
	at at+command3
user can execute following commands:
	select at
	at+command1
	at+command2
	at+command3

where:
at - root command for passing at commands to the modem
at+commandX - at command passed to the modem.

Signed-off-by: Jakub Rzeszutko <jakub.rzeszutko@nordicsemi.no>

include/shell/shell.h

@@ -555,6 +555,9 @@ struct shell_ctx {
 	/*!< Currently executed command.*/
 	struct shell_static_entry active_cmd;
 
+	/* New root command. If NULL shell uses default root commands. */
+	const struct shell_static_entry *selected_cmd;
+
 	/*!< VT100 color and cursor position, terminal width.*/
 	struct shell_vt100_ctx vt100_ctx;