Revert "boot cmds: convert to getenv_yesno() with autostart"

This reverts commit 5a442c0add.

This commit changed the behaviour of getenv_yesno() (both the default
behaviour and the documented behaviour for abbreviated arguments)
which resulted in problems in several areas.

Signed-off-by: Wolfgang Denk <wd@denx.de>

common/cmd_usb.c

@@ -488,7 +488,7 @@ int do_usbboot(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 	flush_cache(addr, (cnt+1)*info.blksz);
 
 	/* Check if we should attempt an auto-start */
-	if (getenv_yesno("autostart")) {
+	if (((ep = getenv("autostart")) != NULL) && (strcmp(ep, "yes") == 0)) {
 		char *local_args[2];
 		local_args[0] = argv[0];
 		local_args[1] = NULL;