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/altera/nios2-generic/u-boot.lds b/board/altera/nios2-generic/u-boot.lds
index d4be077..88ced62 100644
--- a/board/altera/nios2-generic/u-boot.lds
+++ b/board/altera/nios2-generic/u-boot.lds
@@ -104,7 +104,7 @@
 	  *(.scommon)
 	}
 	. = ALIGN(4);
-	_end = .;
+	__bss_end__ = .;
 	PROVIDE (end = .);
 
 	/* DEBUG -- symbol table, string table, etc. etc.