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

This reverts commit 5a442c0addc69d0c4b58e98e5aec1cf07576debb.

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>
diff --git a/common/cmd_nand.c b/common/cmd_nand.c
index c547a68..6783645 100644
--- a/common/cmd_nand.c
+++ b/common/cmd_nand.c
@@ -711,7 +711,7 @@
 			   ulong offset, ulong addr, char *cmd)
 {
 	int r;
-	char *s;
+	char *ep, *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 (getenv_yesno("autostart")) {
+	if (((ep = getenv("autostart")) != NULL) && (strcmp(ep, "yes") == 0)) {
 		char *local_args[2];
 
 		local_args[0] = cmd;