rename _end to __bss_end__

Currently, _end is used for end of BSS section.  We want _end to mean
end of u-boot image, so we rename _end to __bss_end__ first.

Signed-off-by: Po-Yu Chuang <ratbert@faraday-tech.com>
diff --git a/board/actux3/u-boot.lds b/board/actux3/u-boot.lds
index 497ab97..d3463cd 100644
--- a/board/actux3/u-boot.lds
+++ b/board/actux3/u-boot.lds
@@ -71,5 +71,5 @@
 		*(.bss)
 		. = ALIGN(4);
 	}
-	_end =.;
+	__bss_end__ =.;
 }