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/earthlcd/favr-32-ezkit/u-boot.lds b/board/earthlcd/favr-32-ezkit/u-boot.lds
index 0d413a0..20e3631 100644
--- a/board/earthlcd/favr-32-ezkit/u-boot.lds
+++ b/board/earthlcd/favr-32-ezkit/u-boot.lds
@@ -66,5 +66,5 @@
 		*(.bss.*)
 	}
 	. = ALIGN(8);
-	_end = .;
+	__bss_end__ = .;
 }