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_fdos.c b/common/cmd_fdos.c
index 2af4ca0..238abdd 100644
--- a/common/cmd_fdos.c
+++ b/common/cmd_fdos.c
@@ -99,7 +99,7 @@
 	   size, load_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] = argv[0];
 	local_args[1] = NULL;