board: Add a gd flag for chain loading

When U-Boot is run from another boot loader, much of the low-level init
needs to be skipped.

Add a flag for this and adjust ll_boot_init() to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/include/init.h b/include/init.h
index 9ef88c9..b5a167b 100644
--- a/include/init.h
+++ b/include/init.h
@@ -20,7 +20,7 @@
 #ifdef CONFIG_EFI_STUB
 #define ll_boot_init()	false
 #else
-#define ll_boot_init()	true
+#define ll_boot_init()	(!(gd->flags & GD_FLG_SKIP_LL_INIT))
 #endif
 
 /*