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_net.c b/common/cmd_net.c
index c657b03..973fa21 100644
--- a/common/cmd_net.c
+++ b/common/cmd_net.c
@@ -211,7 +211,7 @@
 	flush_cache(load_addr, size);
 
 	/* Loading ok, check if we should attempt an auto-start */
-	if (((s = getenv("autostart")) != NULL) && (strcmp(s,"yes") == 0)) {
+	if (getenv_yesno("autostart")) {
 		char *local_args[2];
 		local_args[0] = argv[0];
 		local_args[1] = NULL;