command: Introduce functions to obtain command arguments

Add some functions which provide an argument to a command, or NULL if
the argument does not exist.

Use the same numbering as argv[] since it seems less confusing than the
previous idea.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>

cmd/disk.c

@@ -40,8 +40,8 @@ int common_diskboot(struct cmd_tbl *cmdtp, const char *intf, int argc,
 
 	bootstage_mark(BOOTSTAGE_ID_IDE_BOOT_DEVICE);
 
-	part = blk_get_device_part_str(intf, (argc == 3) ? argv[2] : NULL,
-					&dev_desc, &info, 1);
+	part = blk_get_device_part_str(intf, cmd_arg2(argc, argv),
+				       &dev_desc, &info, 1);
 	if (part < 0) {
 		bootstage_error(BOOTSTAGE_ID_IDE_TYPE);
 		return 1;