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/fuse.c

@@ -44,7 +44,7 @@ static int confirm_prog(void)
 static int do_fuse(struct cmd_tbl *cmdtp, int flag, int argc,
 		   char *const argv[])
 {
-	const char *op = argc >= 2 ? argv[1] : NULL;
+	const char *op = cmd_arg1(argc, argv);
 	int confirmed = argc >= 3 && !strcmp(argv[2], "-y");
 	u32 bank, word, cnt, val, cmp;
 	ulong addr;