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/trab/u-boot.lds b/board/trab/u-boot.lds
index cd50e85..4594401 100644
--- a/board/trab/u-boot.lds
+++ b/board/trab/u-boot.lds
@@ -60,5 +60,5 @@
 	. = ALIGN(4);
 	__bss_start = .;
 	.bss (NOLOAD) : { *(.bss) . = ALIGN(4); }
-	_end = .;
+	__bss_end__ = .;
 }