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_net.c b/common/cmd_net.c
index 00e0ea8..8c6f5c8 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 (getenv_yesno("autostart")) {
+	if (((s = getenv("autostart")) != NULL) && (strcmp(s,"yes") == 0)) {
 		char *local_args[2];
 		local_args[0] = argv[0];
 		local_args[1] = NULL;