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/gaisler/grsim_leon2/u-boot.lds b/board/gaisler/grsim_leon2/u-boot.lds
index a12e7fb..cf6e502 100644
--- a/board/gaisler/grsim_leon2/u-boot.lds
+++ b/board/gaisler/grsim_leon2/u-boot.lds
@@ -132,7 +132,7 @@
 	. = ALIGN(16); /* to speed clearing of bss up */
 	}
 	__bss_end = . ;
-	_end = . ;
+	__bss_end__ = . ;
 	PROVIDE (end = .);
 
 /* Relocated into main memory */