Make sure that argv[] argument pointers are not modified.

The hush shell dynamically allocates (and re-allocates) memory for the
argument strings in the "char *argv[]" argument vector passed to
commands.  Any code that modifies these pointers will cause serious
corruption of the malloc data structures and crash U-Boot, so make
sure the compiler can check that no such modifications are being done
by changing the code into "char * const argv[]".

This modification is the result of debugging a strange crash caused
after adding a new command, which used the following argument
processing code which has been working perfectly fine in all Unix
systems since version 6 - but not so in U-Boot:

int main (int argc, char **argv)
{
	while (--argc > 0 && **++argv == '-') {
/* ====> */	while (*++*argv) {
			switch (**argv) {
			case 'd':
				debug++;
				break;
			...
			default:
				usage ();
			}
		}
	}
	...
}

The line marked "====>" will corrupt the malloc data structures and
usually cause U-Boot to crash when the next command gets executed by
the shell.  With the modification, the compiler will prevent this with
an
	error: increment of read-only location '*argv'

N.B.: The code above can be trivially rewritten like this:

	while (--argc > 0 && **++argv == '-') {
		char *arg = *argv;
		while (*++arg) {
			switch (*arg) {
			...

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>

include/command.h

@@ -48,14 +48,14 @@ struct cmd_tbl_s {
 	int		maxargs;	/* maximum number of arguments	*/
 	int		repeatable;	/* autorepeat allowed?		*/
 					/* Implementation function	*/
-	int		(*cmd)(struct cmd_tbl_s *, int, int, char *[]);
+	int		(*cmd)(struct cmd_tbl_s *, int, int, char * const []);
 	char		*usage;		/* Usage message	(short)	*/
 #ifdef	CONFIG_SYS_LONGHELP
 	char		*help;		/* Help  message	(long)	*/
 #endif
 #ifdef CONFIG_AUTO_COMPLETE
 	/* do auto completion on the arguments */
-	int		(*complete)(int argc, char *argv[], char last_char, int maxv, char *cmdv[]);
+	int		(*complete)(int argc, char * const argv[], char last_char, int maxv, char *cmdv[]);
 #endif
 };
 
@@ -67,7 +67,7 @@ extern cmd_tbl_t  __u_boot_cmd_end;
 
 /* common/command.c */
 int _do_help (cmd_tbl_t *cmd_start, int cmd_items, cmd_tbl_t * cmdtp, int
-	      flag, int argc, char *argv[]);
+	      flag, int argc, char * const argv[]);
 cmd_tbl_t *find_cmd(const char *cmd);
 cmd_tbl_t *find_cmd_tbl (const char *cmd, cmd_tbl_t *table, int table_len);
 
@@ -83,7 +83,7 @@ extern int cmd_auto_complete(const char *const prompt, char *buf, int *np, int *
  *
  * All commands use a common argument format:
  *
- * void function (cmd_tbl_t *cmdtp, int flag, int argc, char *argv[]);
+ * void function (cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[]);
  */
 
 typedef	void	command_t (cmd_tbl_t *, int, int, char *[]);