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/renesas/sh7757lcr/u-boot.lds b/board/renesas/sh7757lcr/u-boot.lds
index a88d781..790b5b8 100644
--- a/board/renesas/sh7757lcr/u-boot.lds
+++ b/board/renesas/sh7757lcr/u-boot.lds
@@ -97,5 +97,5 @@
 	}
 	PROVIDE (bss_end = .);
 
-	PROVIDE (_end = .);
+	PROVIDE (__bss_end__ = .);
 }