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_nand.c b/common/cmd_nand.c
index 6783645..c547a68 100644
--- a/common/cmd_nand.c
+++ b/common/cmd_nand.c
@@ -711,7 +711,7 @@
 			   ulong offset, ulong addr, char *cmd)
 {
 	int r;
-	char *ep, *s;
+	char *s;
 	size_t cnt;
 	image_header_t *hdr;
 #if defined(CONFIG_FIT)
@@ -787,7 +787,7 @@
 	load_addr = addr;
 
 	/* 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] = cmd;