[cmdline] Generate command option help text automatically

Generate the command option help text automatically from the list of
defined options.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/exec.c

@@ -397,7 +397,7 @@ static struct option_descriptor echo_opts[] = {
 /** "echo" command descriptor */
 static struct command_descriptor echo_cmd =
 	COMMAND_DESC ( struct echo_options, echo_opts, 0, MAX_ARGUMENTS,
-		       "[-n] [...]" );
+		       "[...]" );
 
 /**
  * "echo" command