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/examples/standalone/sparc.lds b/examples/standalone/sparc.lds
index 9733daa..7f060b6 100644
--- a/examples/standalone/sparc.lds
+++ b/examples/standalone/sparc.lds
@@ -57,5 +57,5 @@
 	}
 	. = ALIGN(4);
 	__bss_end = .;
-	_end = .;
+	__bss_end__ = .;
 }