boot cmds: convert to getenv_yesno() with autostart

Use the new helper func to clean up duplicate logic handling of the
autostart env var.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/common/cmd_usb.c b/common/cmd_usb.c
index b04a8df..8439198 100644
--- a/common/cmd_usb.c
+++ b/common/cmd_usb.c
@@ -488,7 +488,7 @@
 	flush_cache(addr, (cnt+1)*info.blksz);
 
 	/* Check if we should attempt an auto-start */
-	if (((ep = getenv("autostart")) != NULL) && (strcmp(ep, "yes") == 0)) {
+	if (getenv_yesno("autostart")) {
 		char *local_args[2];
 		local_args[0] = argv[0];
 		local_args[1] = NULL;