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/arch/arm/cpu/arm720t/u-boot.lds b/arch/arm/cpu/arm720t/u-boot.lds
index 0686e42..deda9fb 100644
--- a/arch/arm/cpu/arm720t/u-boot.lds
+++ b/arch/arm/cpu/arm720t/u-boot.lds
@@ -66,7 +66,7 @@
 		__bss_start = .;
 		*(.bss)
 		 . = ALIGN(4);
-		_end = .;
+		__bss_end__ = .;
 	}
 
 	/DISCARD/ : { *(.dynstr*) }